merge: --no-verify to bypass pre-merge-commit hook

merge: --no-verify to bypass pre-merge-commit hook

Analogous to commit, introduce a '--no-verify' option which bypasses the
pre-merge-commit hook. The shorthand '-n' is taken by '--no-stat'
already.

[js: * reworded commit message to reflect current state of --no-stat flag
       and new hook name
     * fixed flag documentation to reflect new hook name
     * cleaned up trailing whitespace
     * squashed test changes from the original series' patch 4/4
     * modified tests to follow pattern from this series' patch 1/4
     * added a test case for --no-verify with non-executable hook
     * 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).

]

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>