Document receive.denyNonFastforwards

[jc: with a fix to config handling in t5400 test, which took
 annoyingly long to diagnose.]

Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/config.txt

@@ -267,3 +267,10 @@ whatchanged.difftree::
 imap::
 	The configuration variables in the 'imap' section are described
 	in gitlink:git-imap-send[1].
+
+receive.denyNonFastforwads::
+	If set to true, git-receive-pack will deny a ref update which is
+	not a fast forward. Use this to prevent such an update via a push,
+	even if that push is forced. This configuration variable is
+	set when initializing a shared repository.
+

Documentation/git-init-db.txt

@@ -48,6 +48,10 @@ is given:
  - 'all' (or 'world' or 'everybody'): Same as 'group', but make the repository
    readable by all users.
 
+By default, the configuration flag receive.denyNonFastforward is enabled
+in shared repositories, so that you cannot force a non fast-forwarding push
+into it.
+
 --
 
 

Documentation/git-receive-pack.txt

@@ -73,6 +73,8 @@ packed and is served via a dumb transport.
 There are other real-world examples of using update and
 post-update hooks found in the Documentation/howto directory.
 
+git-receive-pack honours the receive.denyNonFastforwards flag, which
+tells it if updates to a ref should be denied if they are not fast-forwards.
 
 OPTIONS
 -------