Contributed bash completion support for core Git tools.

Contributed bash completion support for core Git tools.

This is a set of bash completion routines for many of the
popular core Git tools.  I wrote these routines from scratch
after reading the git-compl and git-compl-lib routines available
from the gitcompletion package at http://gitweb.hawaga.org.uk/
and found those to be lacking in functionality for some commands.
Consequently there may be some similarities but many differences.

Since these are completion routines only for tools shipped with
core Git and since bash is a popular shell on many of the native
core Git platforms (Linux, Mac OS X, Solaris, BSD) including these
routines as part of the stock package would probably be convienent
for many users.

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