Allow whole-tree operations to be started from a subdirectory

Allow whole-tree operations to be started from a subdirectory

This updates five commands (merge, pull, rebase, revert and cherry-pick)
so that they can be started from a subdirectory.

This may not actually be what we want to do.  These commands are
inherently whole-tree operations, and an inexperienced user may
mistakenly expect a "git pull" from a subdirectory would merge
only the subdirectory the command started from.

Signed-off-by: Junio C Hamano <junkio@cox.net>