tree.c: update read_tree_recursive callback to pass strbuf as base

This allows the callback to use 'base' as a temporary buffer to
quickly assemble full path "without" extra allocation. The callback
has to restore it afterwards of course.

Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/merge-recursive.c b/merge-recursive.c
index fdb7d0f..25c067e 100644
--- a/merge-recursive.c
+++ b/merge-recursive.c
@@ -275,23 +275,20 @@
 }
 
 static int save_files_dirs(const unsigned char *sha1,
-		const char *base, int baselen, const char *path,
+		struct strbuf *base, const char *path,
 		unsigned int mode, int stage, void *context)
 {
-	int len = strlen(path);
-	char *newpath = xmalloc(baselen + len + 1);
+	int baselen = base->len;
 	struct merge_options *o = context;
 
-	memcpy(newpath, base, baselen);
-	memcpy(newpath + baselen, path, len);
-	newpath[baselen + len] = '\0';
+	strbuf_addstr(base, path);
 
 	if (S_ISDIR(mode))
-		string_list_insert(&o->current_directory_set, newpath);
+		string_list_insert(&o->current_directory_set, base->buf);
 	else
-		string_list_insert(&o->current_file_set, newpath);
-	free(newpath);
+		string_list_insert(&o->current_file_set, base->buf);
 
+	strbuf_setlen(base, baselen);
 	return (S_ISDIR(mode) ? READ_TREE_RECURSIVE : 0);
 }