Refresh magit buffer delete ongoing commit message

[jreybert]

When refreshing, detect if a commit message is ongoing, and save it. Write it to EDIT_COMMITMSG is not a good lead.

[jreybert]

Use git gui trick:

• while writing a commit message, write it in .git/MAGIT_MSG
• when commit --amend, keep MAGIT_MSG but only show COMMIT_EDITMSG
• discard it when commiting
• think to dump commit message on vimagit events (e.g. refresh) and on some vim events (buffer close...)