Add internal ref set_name fn instead of realloc

The refdb_fs implementation calls realloc directly on a reference
object when it wants to rename it.  It is not a public object, so
this doesn't mess with the immutability of references, but it does
assume certain constraints on the reference representation.  This
commit wraps that assumption in an isolated API to isolate it.

src/refs.c

@@ -88,6 +88,17 @@ git_reference *git_reference__alloc(
 	return ref;
 }
 
+git_reference *git_reference__set_name(
+	git_reference *ref, const char *name)
+{
+	size_t namelen = strlen(name);
+	git_reference *rewrite =
+		git__realloc(ref, sizeof(git_reference) + namelen + 1);
+	if (rewrite != NULL)
+		memcpy(rewrite->name, name, namelen + 1);
+	return rewrite;
+}
+
 void git_reference_free(git_reference *reference)
 {
 	if (reference == NULL)

src/refs.h

@@ -61,6 +61,8 @@ struct git_reference {
 	char name[0];
 };
 
+git_reference *git_reference__set_name(git_reference *ref, const char *name);
+
 int git_reference__normalize_name_lax(char *buffer_out, size_t out_size, const char *name);
 int git_reference__normalize_name(git_buf *buf, const char *name, unsigned int flags);
 int git_reference__update_terminal(git_repository *repo, const char *ref_name, const git_oid *oid);