fix: #895 Image Updater continuously force-pushes new commits when checkout branch is specified

[chengfang]

Fixes #895

This PR changes how a custom git write-back branch is created. Please review and look for any side effects. See issue comments #895 (comment)

Tested with the following scenarios with the following app config:

annotations:
    argocd-image-updater.argoproj.io/image-list: nginx=nginx:1.16.x
    argocd-image-updater.argoproj.io/update-strategy: semver
    argocd-image-updater.argoproj.io/force-update: "false"
    argocd-image-updater.argoproj.io/write-back-method: git:secret:argocd/git-creds
    argocd-image-updater.argoproj.io/git-branch: :push-branch

• initially the write-back branch push-branch does not exist. image-updater creates the new branch from the checkout branch main, and writes the update to the file push-branch-kustomize/source/.argocd-source-argocd_push-branch-kustomize.yaml
• in the next few iterations, the write branch already exists, and there are no updates to the image. So image updater uses the existing write branch push-branch, and no changes are committed or pushed.
• update the application version constraint so that new tags will be available. image updater uses the existing write branch push-branch to commit and push the updates.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 42.85714% with 8 lines in your changes missing coverage. Please review.

Project coverage is 74.70%. Comparing base (78e925b) to head (a325229).

Files with missing lines	Patch %	Lines
pkg/argocd/git.go	42.85%	6 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
- Coverage   74.84%   74.70%   -0.14%     
==========================================
  Files          31       31              
  Lines        3915     3922       +7     
==========================================
  Hits         2930     2930              
- Misses        850      856       +6     
- Partials      135      136       +1     

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[jannfis]

LGTM