git_panel: Load template commit message

[jackjen]

Closes #41371

Release Notes:

• Adds loading commit message from template file set in git config commit.template

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[jackjen]

@cole-miller do you think this needs tests or anything else?

[esthertrapadoux]

Hey @jackjen just following up to let you know the team went pretty deep in more git feature work and we got behind on PR reviews. We're ramping back up and appreciate the patience! 👍