documentation: use the word "index" in the git-commit man page

documentation: use the word "index" in the git-commit man page

As with git-add, I think previous updates to the git-commit man page did
indeed help make it more user-friendly.  But I think the banishment of
the word "index" from the description goes too far; reinstate its use,
to simplify some of the language slightly and smooth the transition to
other documentation.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>