Document receive.denyNonFastforwards

[jc: with a fix to config handling in t5400 test, which took
 annoyingly long to diagnose.]
Signed-off-by: NJohannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: NJunio 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
 -------

t/t5400-send-pack.sh

@@ -64,4 +64,18 @@ test_expect_success \
 	cmp victim/.git/refs/heads/master .git/refs/heads/master
 '
 
+unset GIT_CONFIG GIT_CONFIG_LOCAL
+HOME=`pwd`/no-such-directory
+export HOME ;# this way we force the victim/.git/config to be used.
+
+test_expect_success \
+        'pushing with --force should be denied with denyNonFastforwards' '
+	cd victim &&
+	git-repo-config receive.denyNonFastforwards true &&
+	cd .. &&
+	git-update-ref refs/heads/master master^ &&
+	git-send-pack --force ./victim/.git/ master &&
+	! diff -u .git/refs/heads/master victim/.git/refs/heads/master
+'
+
 test_done