Fix Discard of deleted files

[isc-etamarch]

Fixes #688

[isc-etamarch]

I did not fix stash, since stashing deleted files is frustrating. The best solution would be 'git stash push --staged' which would stash all staged files (just the deleted file), but this seems to break the unstaged files a little, where we now cannot see a diff from the Git UI, and git loses track of the type of operation (i.e. if a file is new, changed, etc.).

Until I find a solution that doesn't break everything, recommendation would be to discard deletions (especially since deleting is an easy operation to redo if needed).

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 35.29412% with 11 lines in your changes missing coverage. Please review.

Project coverage is 40.47%. Comparing base (4f1f989) to head (679ef41).

Files with missing lines	Patch %	Lines
cls/SourceControl/Git/DiscardState.cls	35.29%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #689      +/-   ##
==========================================
- Coverage   40.60%   40.47%   -0.13%     
==========================================
  Files          23       23              
  Lines        3145     3150       +5     
==========================================
- Hits         1277     1275       -2     
- Misses       1868     1875       +7     

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[isc-pbarton]

@isc-etamarch I think this fixes a somewhat different issue than what we saw today that motivated #688. I'm still able to replicate that one by:

• Add a production to source control and commit it.
• Use the "List Productions" page to delete the production
• In the Git repository, run "git add " to add the delete to the index. (This simulates what might happen if I tried to commit the delete through Web UI, the add succeeds, but the commit fails).
• In the Git Web UI, select the deleted file and use the "Discard" option. It fails with "fatal: pathspec 'cls/Diagram/SampleProduction.cls' did not match any files" because it is trying to do a "git add" but the file was already removed from the index.

This goes through a different code path that doesn't involve SourceControl.Git.DiscardState

[isc-etamarch]

@isc-pbarton I see. That last commit should fix that as well (The other changes I made are still necessary for it to work)

[isc-tleavitt]

One really minor nitpick - feel free to tack on a commit to fix, then merge.