Use const qualifier for 'sha1' parameter in delete_ref function

delete_ref function does not change the 'sha1' parameter. Non-const pointer
causes a compiler warning if you call to the function using a const argument.
Signed-off-by: NCarlos Rica <jasampler@gmail.com>
Signed-off-by: NJunio C Hamano <junkio@cox.net>

cache.h

@@ -213,7 +213,7 @@ struct lock_file {
 extern int hold_lock_file_for_update(struct lock_file *, const char *path, int);
 extern int commit_lock_file(struct lock_file *);
 extern void rollback_lock_file(struct lock_file *);
-extern int delete_ref(const char *, unsigned char *sha1);
+extern int delete_ref(const char *, const unsigned char *sha1);
 
 /* Environment bits from configuration mechanism */
 extern int use_legacy_headers;

refs.c

@@ -705,7 +705,7 @@ static int repack_without_ref(const char *refname)
 	return commit_lock_file(&packlock);
 }
 
-int delete_ref(const char *refname, unsigned char *sha1)
+int delete_ref(const char *refname, const unsigned char *sha1)
 {
 	struct ref_lock *lock;
 	int err, i, ret = 0, flag = 0;