sha1-name.c: add `get_oidf()` which acts like `get_oid()`

Compared to `get_oid()`, `get_oidf()` has as parameters
a pointer to `object_id`, a printf format string and
additional arguments. This will help simplify the code
in subsequent commits.

Original-idea-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Paul-Sebastian Ungureanu <ungureanupaulsebastian@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -1333,6 +1333,7 @@ struct object_context {
 	GET_OID_BLOB)
 
 extern int get_oid(const char *str, struct object_id *oid);
+extern int get_oidf(struct object_id *oid, const char *fmt, ...);
 extern int get_oid_commit(const char *str, struct object_id *oid);
 extern int get_oid_committish(const char *str, struct object_id *oid);
 extern int get_oid_tree(const char *str, struct object_id *oid);

sha1-name.c

@@ -1542,6 +1542,25 @@ int get_oid(const char *name, struct object_id *oid)
 	return get_oid_with_context(name, 0, oid, &unused);
 }
 
+/*
+ * This returns a non-zero value if the string (built using printf
+ * format and the given arguments) is not a valid object.
+ */
+int get_oidf(struct object_id *oid, const char *fmt, ...)
+{
+	va_list ap;
+	int ret;
+	struct strbuf sb = STRBUF_INIT;
+
+	va_start(ap, fmt);
+	strbuf_vaddf(&sb, fmt, ap);
+	va_end(ap);
+
+	ret = get_oid(sb.buf, oid);
+	strbuf_release(&sb);
+
+	return ret;
+}
 
 /*
  * Many callers know that the user meant to name a commit-ish by