git-merge: add support for branch.<name>.mergeoptions

This enables per branch configuration of merge options. Currently, the most
useful options to specify per branch are --squash, --summary/--no-summary
and possibly --strategy, but all options are supported.

Note: Options containing whitespace will _not_ be handled correctly. Luckily,
the only option which can include whitespace is --message and it doesn't
make much sense to give that option a default value.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/Documentation/config.txt b/Documentation/config.txt
index 015910f..d3c25f3 100644
--- a/Documentation/config.txt
+++ b/Documentation/config.txt
@@ -337,6 +337,12 @@
 	branch.<name>.merge to the desired branch, and use the special setting
 	`.` (a period) for branch.<name>.remote.
 
+branch.<name>.mergeoptions::
+	Sets default options for merging into branch <name>. The syntax and
+	supported options are equal to that of gitlink:git-merge[1], but
+	option values containing whitespace characters are currently not
+	supported.
+
 clean.requireForce::
 	A boolean to make git-clean do nothing unless given -f or -n.  Defaults
 	to false.