Documentation/git-history: document default for "--update-refs="

While we document the values that can be passed to the "--update-refs="
option, we don't give the user any hint what the default behaviour is.
Document it.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

Documentation/git-history.adoc

@@ -70,7 +70,7 @@ OPTIONS
 	Control which references will be updated by the command, if any. With
 	`branches`, all local branches that point to commits which are
 	descendants of the original commit will be rewritten. With `head`, only
-	the current `HEAD` reference will be rewritten.
+	the current `HEAD` reference will be rewritten. Defaults to `branches`.
 
 GIT
 ---