fix: use correct default branch in multi-source applications

[aali309]

This PR fixes: #932

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.63%. Comparing base (c0ccf02) to head (413a482).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1056      +/-   ##
==========================================
+ Coverage   63.37%   63.63%   +0.26%     
==========================================
  Files          15       15              
  Lines        2326     2343      +17     
==========================================
+ Hits         1474     1491      +17     
  Misses        760      760              
  Partials       92       92              

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[chengfang]

the e2e test is passing because it's using the default argocd writeback, not the git write-back. I think we can skip e2e test for this issue, and some unit tests should suffice.

[chengfang]

we cannot take the annotation value as is, since it can be a composite form like "main:changes". See https://argocd-image-updater.readthedocs.io/en/latest/basics/update-methods/#specifying-a-branch-to-commit-to

I guess we dont' see the above block. Line 200 already handles this case. See next comment.

[chengfang]

if the gitbranch is set, we always take it. So I think we only need to call the func getWriteBackBranch if wbc.GitBranch is empty (in the else clause).