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>

Documentation/git-commit.txt

@@ -8,9 +8,9 @@ git-commit - Record changes to the repository
 SYNOPSIS
 --------
 [verse]
-'git-commit' [-a | --interactive] [-s] [-v] [-u]
-	   [(-c | -C) <commit> | -F <file> | -m <msg> | --amend]
-	   [--allow-empty] [--no-verify] [-e] [--author <author>]
+'git-commit' [-a | --interactive] [-s] [-v] [-u] [--amend]
+	   [(-c | -C) <commit>] [-F <file> | -m <msg>]
+	   [--allow-empty] [--no-verify] [-e] [--author=<author>]
 	   [--cleanup=<mode>] [--] [[-i | -o ]<file>...]
 
 DESCRIPTION
@@ -68,7 +68,7 @@ OPTIONS
 	Take the commit message from the given file.  Use '-' to
 	read the message from the standard input.
 
---author <author>::
+--author=<author>::
 	Override the author name used in the commit.  Use
 	`A U Thor <author@example.com>` format.