CLN: do not use .repeat asv setting for storing benchmark data
`asv` uses `.repeat` to specify the number of times a benchmark should be repeated; our `strings.Replace` benchmark inadvertently uses this to store benchmark data. This doesn't cause issues until after the first parameter:
```
[ 99.87%] ··· strings.Repeat.time_repeat                                                                                                                                                          1/2 failed
[ 99.87%] ··· ========= ===========
               repeats             
              --------- -----------
                 int     151±0.9ms 
                array      failed  
              ========= ===========

[ 99.87%] ···· For parameters: 'array'
               Traceback (most recent call last):
                 File "/home/chris/code/asv/asv/benchmark.py", line 595, in run
                   min_repeat, max_repeat, max_time = self.repeat
               ValueError: too many values to unpack (expected 3)
               
               During handling of the above exception, another exception occurred:
               
               Traceback (most recent call last):
                 File "/home/chris/code/asv/asv/benchmark.py", line 1170, in main_run_server
                   main_run(run_args)
                 File "/home/chris/code/asv/asv/benchmark.py", line 1044, in main_run
                   result = benchmark.do_run()
                 File "/home/chris/code/asv/asv/benchmark.py", line 523, in do_run
                   return self.run(*self._current_params)
                 File "/home/chris/code/asv/asv/benchmark.py", line 597, in run
                   if self.repeat == 0:
               ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()

```
With this PR, both parameters now succeed:
```
[  0.00%] · For pandas commit 8825f78e <repeat>:
[  0.00%] ·· Benchmarking conda-py3.6-Cython-matplotlib-numexpr-numpy-openpyxl-pytables-pytest-scipy-sqlalchemy-xlrd-xlsxwriter-xlwt
[ 50.00%] ··· Running (strings.Repeat.time_repeat--).
[100.00%] ··· strings.Repeat.time_repeat                                                                                                                                                                  ok
[100.00%] ··· ========= ===========
               repeats             
              --------- -----------
                 int      152±1ms  
                array    150±0.6ms 
              ========= ===========
```

- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=h1) Report
> Merging [#25015](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ece58cb39f2afa6dbc0c64f701847f7efd718a58?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25015/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25015      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52401    52401              
==========================================
- Hits        48410    48409       -1     
- Misses       3991     3992       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `42.88% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25015/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=footer). Last update [ece58cb...8825f78](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=h1) Report
> Merging [#25015](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ece58cb39f2afa6dbc0c64f701847f7efd718a58?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25015/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25015      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52401    52401              
==========================================
- Hits        48410    48409       -1     
- Misses       3991     3992       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.8% <ø> (ø)` | :arrow_up: |
| #single | `42.88% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25015/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=footer). Last update [ece58cb...8825f78](https://codecov.io/gh/pandas-dev/pandas/pull/25015?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Thanks @qwhelan !