unique_path: fix unlikely heap overflow

When merge-recursive creates a unique filename, it uses a
template like:

  path~branch_%d

where the final "_%d" is filled by an incrementing counter
until we find a unique name. We allocate 8 characters for
the counter, but there is no logic to limit the size of the
integer.

Of course, this is extremely unlikely, as you would need a
hundred million collisions to trigger the problem.  Even if
an attacker constructed a specialized repo, it is unlikely
that the victim would have the patience to run the merge.

However, we can make it trivially correct (and hopefully
more readable) by using a strbuf.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

merge-recursive.c

@@ -601,25 +601,36 @@ static int remove_file(struct merge_options *o, int clean,
 	return 0;
 }
 
+/* add a string to a strbuf, but converting "/" to "_" */
+static void add_flattened_path(struct strbuf *out, const char *s)
+{
+	size_t i = out->len;
+	strbuf_addstr(out, s);
+	for (; i < out->len; i++)
+		if (out->buf[i] == '/')
+			out->buf[i] = '_';
+}
+
 static char *unique_path(struct merge_options *o, const char *path, const char *branch)
 {
-	char *newpath = xmalloc(strlen(path) + 1 + strlen(branch) + 8 + 1);
+	struct strbuf newpath = STRBUF_INIT;
 	int suffix = 0;
 	struct stat st;
-	char *p = newpath + strlen(path);
-	strcpy(newpath, path);
-	*(p++) = '~';
-	strcpy(p, branch);
-	for (; *p; ++p)
-		if ('/' == *p)
-			*p = '_';
-	while (string_list_has_string(&o->current_file_set, newpath) ||
-	       string_list_has_string(&o->current_directory_set, newpath) ||
-	       lstat(newpath, &st) == 0)
-		sprintf(p, "_%d", suffix++);
+	size_t base_len;
 
-	string_list_insert(&o->current_file_set, newpath);
-	return newpath;
+	strbuf_addf(&newpath, "%s~", path);
+	add_flattened_path(&newpath, branch);
+
+	base_len = newpath.len;
+	while (string_list_has_string(&o->current_file_set, newpath.buf) ||
+	       string_list_has_string(&o->current_directory_set, newpath.buf) ||
+	       lstat(newpath.buf, &st) == 0) {
+		strbuf_setlen(&newpath, base_len);
+		strbuf_addf(&newpath, "_%d", suffix++);
+	}
+
+	string_list_insert(&o->current_file_set, newpath.buf);
+	return strbuf_detach(&newpath, NULL);
 }
 
 static int dir_in_way(const char *path, int check_working_copy)