Create gitmessage file specific to git-mob

[dideler]

Consider having git-mob use a unique gitmessage file rather than the standard filename gitmessage. When creating the file, it copies over the contents of the gitmessage file if present, then appends the co-authors.

Reduces risk, as we can safely delete git-mob's file when soloing. Otherwise it's not clear to git-mob if the file was created by itself or the user.