check_branch_commit(): make first parameter const

Make it clear that this function does not overwrite its first
argument.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -160,7 +160,7 @@ static int branch_merged(int kind, const char *name,
 }
 
 static int check_branch_commit(const char *branchname, const char *refname,
-			       unsigned char *sha1, struct commit *head_rev,
+			       const unsigned char *sha1, struct commit *head_rev,
 			       int kinds, int force)
 {
 	struct commit *rev = lookup_commit_reference(sha1);