Accept and cancel buttons on Git identity panel

[smashwilson]

Please be sure to read the contributor's guide to the GitHub package before submitting any pull requests.

Requirements

• Filling out the template is required. Any pull request that does not include enough information to be reviewed in a timely manner may be closed at the maintainers' discretion.

Description of the Change

Instead of writing your username and email directly to git configuration as you type, use an explicit accept button press. While I'm at it, add separate accept buttons to write to repo-local vs. global git configuration, and add more explanation text to ensure that users know what the control is doing.

The "use for this repository" button is disabled in contexts where there is no local repository.

Screenshot or Gif

Applicable Issues

Fixes #2557.

[codecov]

Codecov Report

Merging #2587 (c5cd601) into master (a747a1b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2587      +/-   ##
==========================================
- Coverage   93.46%   93.45%   -0.01%     
==========================================
  Files         237      237              
  Lines       13231    13234       +3     
  Branches     1906     1906              
==========================================
+ Hits        12366    12368       +2     
- Misses        865      866       +1     

Impacted Files	Coverage Δ
lib/views/git-identity-view.js	100.00% <ø> (ø)
lib/views/git-tab-view.js	86.95% <ø> (ø)
lib/controllers/git-tab-controller.js	86.61% <100.00%> (+0.28%)	⬆️
lib/atom/gutter.js	90.47% <0.00%> (-2.39%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update a747a1b...c5cd601. Read the comment docs.