WIP: grep: remove racy call to repo_clear()

When grepping with --recurse-submodules, grep_submodule() is invoked for
each subrepository to initialize it. At the end of this function,
repo_clear() is called to properly remove the subrepo from memory. When
grepping with threads, however, the call to this function may happen
before the threads have actually finished grepping the subrepo. This
could make Git run into a race condition and possible segmentation
fault. So let's remove the repo_clear() call. This is not the best
solution as it can degrade memory. But to properly handle the subrepo
cleaning, we would need to keep a mapping of subrepos' identifiers to
the number of objects not grepped yet, or something like this.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>

builtin/grep.c

@@ -482,7 +482,12 @@ static int grep_submodule(struct grep_opt *opt,
 		hit = grep_cache(&subopt, pathspec, cached);
 	}
 
-	repo_clear(&subrepo);
+	/*
+	 * NEEDSWORK: The subrepo should be cleaned after use, calling
+	 * repo_clear(&subrepo). It's not possible to do it here, however, as
+	 * threads might still be working on the subrepo's objects.
+	 */
+
 	return hit;
 }