Add git-mv

Add git-mv

It supersedes git-rename by adding functionality to move multiple
files, directories or symlinks into another directory.  It also
provides according documentation.

The implementation renames multiple files, using the arguments from
the command line to produce an array of sources and destinations.  In
a first pass, all requested renames are checked for errors, and
overwriting of existing files is only allowed with '-f'.  The actual
renaming is done in a second pass.  This ensures that any error
condition is checked before anything is changed.

Signed-off-by: Josef Weidendorfer <Josef.Weidendorfer@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>