Allow checkout -B <current-branch> to update the current branch

When on master, "git checkout -B master <commit>" is a more natural way to
say "git reset --keep <commit>", which was originally invented for the
exact purpose of moving to the named commit while keeping the local changes
around.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.h

@@ -13,7 +13,8 @@
  * branch for (if any).
  */
 void create_branch(const char *head, const char *name, const char *start_name,
-		   int force, int reflog, enum branch_track track);
+		   int force, int reflog,
+		   int clobber_head, enum branch_track track);
 
 /*
  * Validates that the requested branch may be created, returning the