refs: fix duplicated word in comment

Fix a typo in a comment in refs.c: "checking checking" → "checking".
Signed-off-by: Christian Fredrik Johnsen <[email protected]>

Author: ChristianFredrikJohnsen

refs.c

@@ -2549,7 +2549,7 @@ 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
+			 * process it again. Skip it to avoid checking
 			 * common prefixes like "refs/heads/" repeatedly.
 			 */
 			if (!strset_add(&dirnames, dirname.buf))