builtin/difftool: intialize some hashmap variables

When running a dir-diff command that produces no diff, variables
`wt_modified` and `tmp_modified` are used while uninitialized, causing:

    $ /home/smarchi/src/git/git-difftool --dir-diff master
    free(): invalid pointer
    [1]    334004 IOT instruction (core dumped)  /home/smarchi/src/git/git-difftool --dir-diff master
    $ valgrind --track-origins=yes /home/smarchi/src/git/git-difftool --dir-diff master
    ...
    Invalid free() / delete / delete[] / realloc()
       at 0x48478EF: free (vg_replace_malloc.c:989)
       by 0x422CAC: hashmap_clear_ (hashmap.c:208)
       by 0x283830: run_dir_diff (difftool.c:667)
       by 0x284103: cmd_difftool (difftool.c:801)
       by 0x238E0F: run_builtin (git.c:484)
       by 0x2392B9: handle_builtin (git.c:750)
       by 0x2399BC: cmd_main (git.c:921)
       by 0x356FEF: main (common-main.c:64)
     Address 0x1ffefff180 is on thread 1's stack
     in frame #2, created by run_dir_diff (difftool.c:358)
    ...

If taking any `goto finish` path before these variables are initialized,
`hashmap_clear_and_free()` operates on uninitialized data, sometimes
causing a crash.

This regression was introduced in 7f795a1 (builtin/difftool: plug
several trivial memory leaks, 2024-09-26).

Fix it by initializing those variables with the `HASHMAP_INIT` macro.

Add a test comparing the main branch to itself, resulting in no diff.

Signed-off-by: Simon Marchi <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

builtin/difftool.c

@@ -376,7 +376,8 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	struct checkout lstate, rstate;
 	int err = 0;
 	struct child_process cmd = CHILD_PROCESS_INIT;
-	struct hashmap wt_modified, tmp_modified;
+	struct hashmap wt_modified = HASHMAP_INIT(path_entry_cmp, NULL);
+	struct hashmap tmp_modified = HASHMAP_INIT(path_entry_cmp, NULL);
 	int indices_loaded = 0;
 
 	workdir = repo_get_work_tree(the_repository);
@@ -601,9 +602,6 @@ static int run_dir_diff(const char *extcmd, int symlinks, const char *prefix,
 	 * in the common case of --symlinks and the difftool updating
 	 * files through the symlink.
 	 */
-	hashmap_init(&wt_modified, path_entry_cmp, NULL, wtindex.cache_nr);
-	hashmap_init(&tmp_modified, path_entry_cmp, NULL, wtindex.cache_nr);
-
 	for (i = 0; i < wtindex.cache_nr; i++) {
 		struct hashmap_entry dummy;
 		const char *name = wtindex.cache[i]->name;

t/t7800-difftool.sh

@@ -666,6 +666,10 @@ run_dir_diff_test 'difftool --dir-diff syncs worktree without unstaged change' '
 	test_cmp expect file
 '
 
+run_dir_diff_test 'difftool --dir-diff with no diff' '
+	git difftool -d main main
+'
+
 write_script modify-file <<\EOF
 echo "new content" >file
 EOF