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