git-commit: document --amend

The "--amend" option is used to amend the tip of the current branch. This
documentation text was copied straight from the commit that implemented it.

Some minor format tweaks for asciidoc were taken from work by Francis Daly
in commit b0d08a5.. It looks good now also in the html page.

[jc: amended further to follow the recommendation by Francis in
commit 3070b60].

Signed-off-by: Marco Roeland <marco.roeland@xs4all.nl>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-commit.txt

@@ -9,7 +9,8 @@ SYNOPSIS
 --------
 [verse]
 'git-commit' [-a] [-s] [-v] [(-c | -C) <commit> | -F <file> | -m <msg>]
-	   [-e] [--author <author>] [--] [[-i | -o ]<file>...]
+	   [--no-verify] [--amend] [-e] [--author <author>]
+	   [--] [[-i | -o ]<file>...]
 
 DESCRIPTION
 -----------
@@ -71,6 +72,27 @@ OPTIONS
 	commit log message unmodified.  This option lets you
 	further edit the message taken from these sources.
 
+--amend::
+
+	Used to amend the tip of the current branch. Prepare the tree
+	object you would want to replace the latest commit as usual
+	(this includes the usual -i/-o and explicit paths), and the
+	commit log editor is seeded with the commit message from the
+	tip of the current branch. The commit you create replaces the
+	current tip -- if it was a merge, it will have the parents of
+	the current tip as parents -- so the current top commit is
+	discarded.
++
+It is a rough equivalent for:
++
+------------
+	$ git reset --soft HEAD^
+	$ ... do something else to come up with the right tree ...
+	$ git commit -c ORIG_HEAD
+------------
++
+but can be used to amend a merge commit.
+
 -i|--include::
 	Instead of committing only the files specified on the
 	command line, update them in the index file and then