strbuf_branchname(): a wrapper for branch name shorthands

The function takes a user-supplied string that is supposed to be a branch
name, and puts it in a strbuf after expanding possible shorthand notation.

A handful of open coded sequence to do this in the existing code have been
changed to use this helper function.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

strbuf.c

@@ -357,3 +357,12 @@ int strbuf_read_file(struct strbuf *sb, const char *path, size_t hint)
 
 	return len;
 }
+
+int strbuf_branchname(struct strbuf *sb, const char *name)
+{
+	int len = strlen(name);
+	if (interpret_branch_name(name, sb) == len)
+		return 0;
+	strbuf_add(sb, name, len);
+	return len;
+}