[PATCH v3 2/3] githooks.txt: Amend dangerous advice about 'update' hook ACL

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

Any ACL you implement via an 'update' hook isn't actual access control
if the user has login access to the machine running git, because they
can trivially just built their own git version which doesn't run the
hook.

Change the documentation to take this dangerous edge case into account,
and remove the mention of the advice originating on the mailing list,
the users reading this don't care where the idea came up.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
---
 Documentation/githooks.txt | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 6db515e..38bea7d 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -275,9 +275,13 @@ does not know the entire set of branches, so it would end up
 firing one e-mail per ref when used naively, though.  The
 <<post-receive,'post-receive'>> hook is more suited to that.
 
-Another use suggested on the mailing list is to use this hook to
-implement access control which is finer grained than the one
-based on filesystem group.
+Another use for this hook to implement access control which is finer
+grained than the one based on filesystem group. Note that if the user
+pushing has a normal login shell on the machine receiving the push
+implementing access control like this can be trivially bypassed by
+just not executing the hook. In those cases consider using
+e.g. linkgit:git-shell[1] as the login shell to restrict the user's
+access.
 
 Both standard output and standard error output are forwarded to
 'git send-pack' on the other end, so you can simply `echo` messages
-- 
2.1.3