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>

Documentation/merge-options.txt

@@ -10,6 +10,10 @@
 	not autocommit, to give the user a chance to inspect and
 	further tweak the merge result before committing.
 
+--commit::
+	Perform the merge and commit the result. This option can
+	be used to override --no-commit.
+
 --squash::
 	Produce the working tree and index state as if a real
 	merge happened, but do not actually make a commit or
@@ -19,6 +23,10 @@
 	top of the current branch whose effect is the same as
 	merging another branch (or more in case of an octopus).
 
+--no-squash::
+	Perform the merge and commit the result. This option can
+	be used to override --squash.
+
 -s <strategy>, \--strategy=<strategy>::
 	Use the given merge strategy; can be supplied more than
 	once to specify them in the order they should be tried.