git-merge: add support for --commit and --no-squash

git-merge: add support for --commit and --no-squash

These options can be used to override --no-commit and --squash, which is
needed since --no-commit and --squash now can be specified as default merge
options in $GIT_DIR/config.

The change also introduces slightly different behavior for --no-commit:
when specified, it explicitly overrides --squash. Earlier,
'git merge --squash --no-commit' would result in a squashed merge (i.e. no
$GIT_DIR/MERGE_HEAD was created) but with this patch the command will
behave as if --squash hadn't been specified.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>