builtin/difftool: intialize some hashmap variables

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 7f795a1715 (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 <simon.marchi@efficios.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>