git-merge: honor pre-merge-commit hook

git-merge does not honor the pre-commit hook when doing automatic merge
commits, and for compatibility reasons this is going to stay.

Introduce a pre-merge-commit hook which is called for an automatic merge
commit just like pre-commit is called for a non-automatic merge commit
(or any other commit).

[js: * renamed hook from "pre-merge" to "pre-merge-commit"
     * only discard the index if the hook is actually present
     * expanded githooks documentation entry
     * clarified that hook should write messages to stderr
     * squashed test changes from the original series' patch 4/4
     * modified tests to follow new pattern from this series' patch 1/4
     * added a test case for non-executable merge hooks
     * added a test case for failed merges
     * when testing that the merge hook did not run, make sure we
       actually have a merge to perform (by resetting the "side" branch
       to its original state).
     * reworded commit message
]

Improved-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Michael J Gruber <git@grubix.eu>
Signed-off-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

templates/hooks--pre-merge-commit.sample

@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+# An example hook script to verify what is about to be committed.
+# Called by "git merge" with no arguments.  The hook should
+# exit with non-zero status after issuing an appropriate message to
+# stderr if it wants to stop the merge commit.
+#
+# To enable this hook, rename this file to "pre-merge-commit".
+
+. git-sh-setup
+test -x "$GIT_DIR/hooks/pre-commit" &&
+        exec "$GIT_DIR/hooks/pre-commit"
+: