refs: break out ref conflict checks

Create new function find_descendant_ref, to hold one of the ref
conflict checks used in verify_refname_available. Multiple backends
will need this function, so move it to the common code.

Also move rename_ref_available to the common code, because alternate
backends might need it and it has no files-backend-specific code.

Signed-off-by: David Turner <dturner@twopensource.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Jeff King <peff@peff.net>

refs/refs-internal.h

@@ -181,4 +181,20 @@ int files_log_ref_write(const char *refname, const unsigned char *old_sha1,
 			const unsigned char *new_sha1, const char *msg,
 			int flags, struct strbuf *err);
 
+/*
+ * Check for entries in extras that are within the specified
+ * directory, where dirname is a reference directory name including
+ * the trailing slash (e.g., "refs/heads/foo/"). Ignore any
+ * conflicting references that are found in skip. If there is a
+ * conflicting reference, return its name.
+ *
+ * extras and skip must be sorted lists of reference names. Either one
+ * can be NULL, signifying the empty list.
+ */
+const char *find_descendant_ref(const char *dirname,
+				const struct string_list *extras,
+				const struct string_list *skip);
+
+int rename_ref_available(const char *oldname, const char *newname);
+
 #endif /* REFS_REFS_INTERNAL_H */