Use .git/MERGE_MSG in cherry-pick/revert

Use .git/MERGE_MSG in cherry-pick/revert

Rather than storing the temporary commit message data in .msg (in
the working tree) we now store the message data in .git/MERGE_MSG.

By storing the message in the .git/ directory we are sure we will
never have a collision with a user file, should a project actually
have a ".msg" file in their top level tree.  We also don't need to
worry about leaving this stale file behind during a `reset --hard`
and have it show up in the output of status.

We are using .git/MERGE_MSG here to store the temporary message as
it is an already established convention between git-merge, git-am
and git-rebase that git-commit will default the user's edit buffer
to the contents of .git/MERGE_MSG.  If the user is going to need
to resolve this commit or wants to edit the message on their own
prepping that file with the desired message "just works".

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>