Wrong default write branch is used in multi-source app

[chengfang]

Describe the bug
In a multi-source application, when a custom write-back git repo is specified, but the git branch is not specified, image-updater selects the branch from the primary source, instead of the targetRevison of the chosen source.

To Reproduce

    argocd-image-updater.argoproj.io/nginx.helm.image-name: image.repository
    argocd-image-updater.argoproj.io/nginx.helm.image-tag: image.tag
    argocd-image-updater.argoproj.io/git-repository: https://github.com/chengfang/image-updater-examples.git
    # argocd-image-updater.argoproj.io/git-branch: main
    argocd-image-updater.argoproj.io/write-back-method: git:secret:argocd/git-creds
    argocd-image-updater.argoproj.io/write-back-target: helmvalues:/write-helmvalues/source2/values.yaml

Expected behavior
the branch of the specified source repo is selected as the default write-back git branch, unless overridden with git-branch annotation.

Additional context
The workaround is to always specify git-branch annotation.

Version
v0.15.1

Logs

INFO[0001] Committing 1 parameter update(s) for application write-helmvalues  application=write-helmvalues
INFO[0001] Initializing https://github.com/chengfang/image-updater-examples.git to /var/folders/sc/bblzy4t15995h2zs1s4bztc00000gn/T/git-write-helmvalues2791939777
INFO[0001] git fetch origin 18.2.3 --force --prune --depth 1  dir=/var/folders/sc/bblzy4t15995h2zs1s4bztc00000gn/T/git-write-helmvalues2791939777 execID=a6d31
ERRO[0002] `git fetch origin 18.2.3 --force --prune --depth 1` failed exit status 128: fatal: couldn't find remote ref 18.2.3  execID=a6d31
INFO[0002] Trace                                         args="[git fetch origin 18.2.3 --force --prune --depth 1]" dir=/var/folders/sc/bblzy4t15995h2zs1s4bztc00000gn/T/git-write-helmvalues2791939777 operation_name="exec git" time_ms=284.635333
ERRO[0002] Could not update application spec: `git fetch origin 18.2.3 --force --prune --depth 1` failed exit status 128: fatal: couldn't find remote ref 18.2.3  application=write-helmvalues
INFO[0002] Processing results: applications=1 images_considered=1 images_skipped=0 images_updated=0 errors=1

[chengfang]

argocd-image-updater/pkg/argocd/git.go

Lines 161 to 165 in 1924339

	// The branch to checkout is either a configured branch in the write-back
	// config, or taken from the application spec's targetRevision. If the
	// target revision is set to the special value HEAD, or is the empty
	// string, we'll try to resolve it to a branch name.
	checkOutBranch := getApplicationSource(app).TargetRevision

Need to improve this block to choose a source matching the git-repository annotation value. If not found, then default to the primary source.