refs: introduce function to batch refname availability checks

The `refs_verify_refname_available()` functions checks whether a
reference update can be committed or whether it would conflict with
either a prefix or suffix thereof. This function needs to be called once
per reference that one wants to check, which requires us to redo a
couple of checks every time the function is called.

Introduce a new function `refs_verify_refnames_available()` that does
the same, but for a list of references. For now, the new function uses
the exact same implementation, except that we loop through all refnames
provided by the caller. This will be tuned in subsequent commits.

The existing `refs_verify_refname_available()` function is reimplemented
on top of the new function. As such, the diff is best viewed with the
`--ignore-space-change option`.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

refs.h

@@ -124,6 +124,18 @@ int refs_verify_refname_available(struct ref_store *refs,
 				  unsigned int initial_transaction,
 				  struct strbuf *err);
 
+/*
+ * Same as `refs_verify_refname_available()`, but checking for a list of
+ * refnames instead of only a single item. This is more efficient in the case
+ * where one needs to check multiple refnames.
+ */
+int refs_verify_refnames_available(struct ref_store *refs,
+				   const struct string_list *refnames,
+				   const struct string_list *extras,
+				   const struct string_list *skip,
+				   unsigned int initial_transaction,
+				   struct strbuf *err);
+
 int refs_ref_exists(struct ref_store *refs, const char *refname);
 
 int should_autocreate_reflog(enum log_refs_config log_all_ref_updates,