delete_ref(): move declaration to refs.h

Also

* Add a docstring

* Rename the second parameter to "old_sha1", to be consistent with the
  convention used elsewhere in the refs module

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

cache.h

@@ -585,8 +585,6 @@ extern void update_index_if_able(struct index_state *, struct lock_file *);
 extern int hold_locked_index(struct lock_file *, int);
 extern void set_alternate_index_output(const char *);
 
-extern int delete_ref(const char *, const unsigned char *sha1, unsigned int flags);
-
 /* Environment bits from configuration mechanism */
 extern int trust_executable_bit;
 extern int trust_ctime;

refs.c

@@ -2801,7 +2801,8 @@ static int delete_ref_loose(struct ref_lock *lock, int flag, struct strbuf *err)
 	return 0;
 }
 
-int delete_ref(const char *refname, const unsigned char *sha1, unsigned int flags)
+int delete_ref(const char *refname, const unsigned char *old_sha1,
+	       unsigned int flags)
 {
 	struct ref_transaction *transaction;
 	struct strbuf err = STRBUF_INIT;
@@ -2809,7 +2810,7 @@ int delete_ref(const char *refname, const unsigned char *sha1, unsigned int flag
 	transaction = ref_transaction_begin(&err);
 	if (!transaction ||
 	    ref_transaction_delete(transaction, refname,
-				   (sha1 && !is_null_sha1(sha1)) ? sha1 : NULL,
+				   (old_sha1 && !is_null_sha1(old_sha1)) ? old_sha1 : NULL,
 				   flags, NULL, &err) ||
 	    ref_transaction_commit(transaction, &err)) {
 		error("%s", err.buf);

refs.h

@@ -202,6 +202,16 @@ extern int read_ref_at(const char *refname, unsigned int flags,
 /** Check if a particular reflog exists */
 extern int reflog_exists(const char *refname);
 
+/*
+ * Delete the specified reference. If old_sha1 is non-NULL and not
+ * NULL_SHA1, then verify that the current value of the reference is
+ * old_sha1 before deleting it. If old_sha1 is NULL or NULL_SHA1,
+ * delete the reference if it exists, regardless of its old value.
+ * flags is passed through to ref_transaction_delete().
+ */
+extern int delete_ref(const char *refname, const unsigned char *old_sha1,
+		      unsigned int flags);
+
 /** Delete a reflog */
 extern int delete_reflog(const char *refname);