Only show identity on present repositories

[smashwilson]

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Description of the Change

The git identity panel introduced in #2428 flips on automatically when the username or email are not set in your git config, but an empty (uninitialized) or absent (no working directories) repository would never have either. Let's only show the git identity panel when the repository is actually present.

Applicable Issues

Fixes #2435.

[codecov]

Codecov Report

Merging #2437 into master will increase coverage by 0.31%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2437      +/-   ##
==========================================
+ Coverage   93.11%   93.43%   +0.31%     
==========================================
  Files         236      236              
  Lines       13196    13196              
  Branches     1897     1897              
==========================================
+ Hits        12288    12330      +42     
+ Misses        908      866      -42     

Impacted Files	Coverage Δ
lib/controllers/git-tab-controller.js	85.71% <ø> (ø)
lib/git-shell-out-strategy.js	99.81% <0.00%> (+0.56%)	⬆️
lib/atom/gutter.js	92.85% <0.00%> (+2.38%)	⬆️
lib/helpers.js	89.30% <0.00%> (+2.88%)	⬆️
lib/models/file-system-change-observer.js	96.00% <0.00%> (+8.00%)	⬆️
lib/shared/keytar-strategy.js	58.13% <0.00%> (+19.37%)	⬆️
lib/models/repository-states/empty.js	100.00% <0.00%> (+22.22%)	⬆️

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