Teach merge the '[-e|--edit]' option

Teach merge the '[-e|--edit]' option

Implemented internally instead of as "git merge --no-commit && git commit"
so that "merge --edit" is otherwise consistent (hooks, etc) with "merge".

Note: the edit message does not include the status information that one
gets with "commit --status" and it is cleaned up after editing like one
gets with "commit --cleanup=default". A later patch could add the status
information if desired.

Note: previously we were not calling stripspace() after running the
prepare-commit-msg hook. Now we are, stripping comments and
leading/trailing whitespace lines if --edit is given, otherwise only
stripping leading/trailing whitespace lines if not given --edit.

Signed-off-by: Jay Soffian <jaysoffian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>