BUG: pd.concat does not drop DatetimeIndex.freq when result is monotonic with even spacing
- [x] closes #3232
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25796?src=pr&el=h1) Report
> Merging [#25796](https://codecov.io/gh/pandas-dev/pandas/pull/25796?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/46639512c06300a9844ea27f90167d5648c9b93a?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25796/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25796?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25796      +/-   ##
==========================================
- Coverage   91.26%   91.26%   -0.01%     
==========================================
  Files         173      173              
  Lines       52982    52983       +1     
==========================================
  Hits        48356    48356              
- Misses       4626     4627       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.83% <100%> (ø)` | :arrow_up: |
| #single | `41.76% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25796?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/25796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `98.52% <100%> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/25796/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `89.3% <0%> (-0.11%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25796?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/25796?src=pr&el=footer). Last update [4663951...f7e087d](https://codecov.io/gh/pandas-dev/pandas/pull/25796?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/25796?src=pr&el=h1) Report
> Merging [#25796](https://codecov.io/gh/pandas-dev/pandas/pull/25796?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d37c531075322b4aa115f5f9f0cb9173773afbdc?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/25796/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/25796?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #25796      +/-   ##
==========================================
+ Coverage   91.48%   91.48%   +<.01%     
==========================================
  Files         175      175              
  Lines       52877    52879       +2     
==========================================
+ Hits        48372    48374       +2     
  Misses       4505     4505
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.04% <100%> (ø)` | :arrow_up: |
| #single | `41.83% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/25796?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/datetimelike.py](https://codecov.io/gh/pandas-dev/pandas/pull/25796/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZWxpa2UucHk=) | `98.53% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/25796?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/25796?src=pr&el=footer). Last update [d37c531...da62bde](https://codecov.io/gh/pandas-dev/pandas/pull/25796?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. merge on green. thanks @mroeschke 