git-commit.txt: Correct option alternatives

git-commit.txt: Correct option alternatives

This patch fixes the SYNOPSIS in git-commit.txt:

 * --amend could be used in conjunction with -c/-C/-F/-m;
   it is not mutually exclusive with them.

 * -m and -F are not alternative options to -c/-C;
   you can reuse authorship from a commit (-c/-C)
   but change the message (-m/-F).

Furthermore, for long-option consistency --author <author>
is changed to --author=<author>.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>