post-merge: Add it's not executed if merge failed.

Signed-off-by: NJ��rg Sommer <joerg@alea.gnuu.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/hooks.txt

@@ -137,7 +137,8 @@ post-merge
 This hook is invoked by `git-merge`, which happens when a `git pull`
 is done on a local repository.  The hook takes a single parameter, a status
 flag specifying whether or not the merge being done was a squash merge.
-This hook cannot affect the outcome of `git-merge`.
+This hook cannot affect the outcome of `git-merge` and is not executed,
+if the merge failed due to conflicts.
 
 This hook can be used in conjunction with a corresponding pre-commit hook to
 save and restore any form of metadata associated with the working tree