feat: push new commit to pushBranch when newer image is found

[katainaka0503]

If pushBranch is designated like :target_branch, image updater continues to emit error messages like below after pushing a branch initially.

time="2022-03-29T11:03:47Z" level=error msg="Could not update application spec: could not push image-updater-katainaka0503/argocd-image-updater-test-app-0.0.4 to origin: `git push origin image-updater-katainaka0503/argocd-image-updater-test-app-0.0.4` failed exit status 1: To https://github.com/katainaka0503/argocd-image-updater-test.git\n ! [rejected]        image-updater-katainaka0503/argocd-image-updater-test-app-0.0.4 -> image-updater-katainaka0503/argocd-image-updater-test-app-0.0.4 (non-fast-forward)\nerror: failed to push some refs to 'https://github.com/katainaka0503/argocd-image-updater-test.git'\nhint: Updates were rejected because the tip of your current branch is behind\nhint: its remote counterpart. Integrate the remote changes (e.g.\nhint: 'git pull ...') before pushing again.\nhint: See the 'Note about fast-forwards' in 'git push --help' for details." application=test

This PR allows the image updater to update the branch if designated branch is found on the remote repository.

Related: #304, #384

[katainaka0503]

@jannfis Hi! Is there anything I can to help you review this PR?

[codecov-commenter]

Codecov Report

Merging #410 (760bc66) into master (ccb749f) will increase coverage by 0.01%.
The diff coverage is 75.00%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   65.70%   65.72%   +0.01%     
==========================================
  Files          21       21              
  Lines        2041     2048       +7     
==========================================
+ Hits         1341     1346       +5     
- Misses        571      572       +1     
- Partials      129      130       +1     

Impacted Files	Coverage Δ
pkg/argocd/git.go	65.04% <75.00%> (+0.20%)	⬆️

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