merge-ort: reuse path strings in pool_alloc_filespec

pool_alloc_filespec() was written so that the code when pool != NULL
mimicked the code from alloc_filespec(), which including allocating
enough extra space for the path and then copying it.  However, the path
passed to pool_alloc_filespec() is always going to already be in the
same memory pool, so we may as well reuse it instead of copying it.

For the testcases mentioned in commit 557ac03 ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:       198.5 ms ±  3.4 ms     198.3 ms ±  2.9 ms
    mega-renames:     679.1 ms ±  5.6 ms     661.8 ms ±  5.9 ms
    just-one-mega:    271.9 ms ±  2.8 ms     264.6 ms ±  2.5 ms

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-ort.c

@@ -694,17 +694,13 @@ static struct diff_filespec *pool_alloc_filespec(struct mem_pool *pool,
 						 const char *path)
 {
 	struct diff_filespec *spec;
-	size_t len;
 
 	if (!pool)
 		return alloc_filespec(path);
 
-	/* Same code as alloc_filespec, except allocate from pool */
-	len = strlen(path);
-
-	spec = mem_pool_calloc(pool, 1, st_add3(sizeof(*spec), len, 1));
-	memcpy(spec+1, path, len);
-	spec->path = (void*)(spec+1);
+	/* Similar to alloc_filespec, but allocate from pool and reuse path */
+	spec = mem_pool_calloc(pool, 1, sizeof(*spec));
+	spec->path = (char*)path; /* spec won't modify it */
 
 	spec->count = 1;
 	spec->is_binary = -1;
@@ -2904,6 +2900,25 @@ static void use_cached_pairs(struct merge_options *opt,
 		const char *new_name = entry->value;
 		if (!new_name)
 			new_name = old_name;
+		if (pool) {
+			/*
+			 * cached_pairs has _copies* of old_name and new_name,
+			 * because it has to persist across merges.  When
+			 *   pool != NULL
+			 * pool_alloc_filespec() will just re-use the existing
+			 * filenames, which will also get re-used by
+			 * opt->priv->paths if they become renames, and then
+			 * get freed at the end of the merge, leaving the copy
+			 * in cached_pairs dangling.  Avoid this by making a
+			 * copy here.
+			 *
+			 * When pool == NULL, pool_alloc_filespec() calls
+			 * alloc_filespec(), which makes a copy; we don't want
+			 * to add another.
+			 */
+			old_name = mem_pool_strdup(pool, old_name);
+			new_name = mem_pool_strdup(pool, new_name);
+		}
 
 		/* We don't care about oid/mode, only filenames and status */
 		one = pool_alloc_filespec(pool, old_name);