refs: fix duplicated word in comment

Fix a typo in a comment in refs.c: "checking checking" → "checking".

Signed-off-by: Christian Fredrik Johnsen <christian@johnsen.no>
Acked-by: Martin Ågren <martin.agren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: ChristianFredrikJohnsen

refs.c

@@ -2529,8 +2529,8 @@ int refs_verify_refnames_available(struct ref_store *refs,
 
 			/*
 			 * If we've already seen the directory we don't need to
-			 * process it again. Skip it to avoid checking checking
-			 * common prefixes like "refs/heads/" repeatedly.
+			 * process it again. Skip it to avoid checking common
+			 * prefixes like "refs/heads/" repeatedly.
 			 */
 			if (!strset_add(&dirnames, dirname.buf))
 				continue;