BUG: show series length in repr when truncated (GH15962)
Also follow the display.show_dimensions option.

 - [x] closes #15962, closes #7117
 - [x] tests added / passed
 - [ ] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [ ] whatsnew entry

lgtm. @jorisvandenbossche can you update # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=h1) Report
> Merging [#15974](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cd1031f28664928c7e258dbb04d1f5861bad6b37?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15974/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15974      +/-   ##
==========================================
+ Coverage   90.99%      91%   +<.01%     
==========================================
  Files         153      153              
  Lines       50481    50478       -3     
==========================================
- Hits        45937    45935       -2     
+ Misses       4544     4543       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <66.66%> (ø)` | :arrow_up: |
| #single | `40.43% <55.55%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `95.11% <100%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `95.07% <62.5%> (-0.02%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.73% <0%> (+0.18%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15974?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/15974?src=pr&el=footer). Last update [cd1031f...daf01f7](https://codecov.io/gh/pandas-dev/pandas/pull/15974?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/15974?src=pr&el=h1) Report
> Merging [#15974](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/cd1031f28664928c7e258dbb04d1f5861bad6b37?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15974/graphs/tree.svg?src=pr&width=650&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15974      +/-   ##
==========================================
+ Coverage   90.99%      91%   +<.01%     
==========================================
  Files         153      153              
  Lines       50481    50478       -3     
==========================================
- Hits        45937    45935       -2     
+ Misses       4544     4543       -1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.76% <66.66%> (ø)` | :arrow_up: |
| #single | `40.43% <55.55%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `95.11% <100%> (ø)` | :arrow_up: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `95.07% <62.5%> (-0.02%)` | :arrow_down: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `90.68% <0%> (-0.35%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `80.73% <0%> (+0.18%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15974?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/15974?src=pr&el=footer). Last update [cd1031f...daf01f7](https://codecov.io/gh/pandas-dev/pandas/pull/15974?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
