Both remove_branch_state() and drop_save() delete almost the same set of
files about the current merge state. The only difference is MERGE_RR but
it should also be cleaned up after a successful merge, which is what
drop_save() is for.

Make a new function that deletes all merge-related state files and use
it instead of drop_save(). This function will also be used in the next
patch that introduces --quit.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

This allows to cancel the current merge without resetting worktree/index,
which is what --abort is for. Like other --quit(s), this is often used
when you forgot that you're in the middle of a merge and already
switched away, doing different things. By the time you've realized, you
can't even continue the merge anymore.

This also makes all in-progress commands, am, merge, rebase, revert and
cherry-pick, take all three --abort, --continue and --quit (bisect has a
different UI).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>