git: Fill the commit message buffer from MERGE_MSG (#24843)

This PR uses the template merge message in `.git/MERGE_MSG` to populate
the commit message buffer in the git panel. This is done:

- when the commit message buffer is first created
- when the list of merge heads in .git changes, only if the buffer
doesn't already have some text in it

Hopefully this strikes a good balance between convenience and not
stomping on the user's toes.

Release Notes:

- N/A