ls-files: free max_prefix when done

ls-files: free max_prefix when done

common_prefix() returns a new string, which we store in max_prefix -
this string needs to be freed to avoid a leak. This leak is happening
in cmd_ls_files, hence is of no real consequence - an UNLEAK would be
just as good, but we might as well free the string properly.

Leak found while running t0002, see output below:

Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x49a85d in malloc /home/abuild/rpmbuild/BUILD/llvm-11.0.0.src/build/../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:145:3
    #1 0x9ab1b4 in do_xmalloc wrapper.c:41:8
    #2 0x9ab248 in do_xmallocz wrapper.c:75:8
    #3 0x9ab22a in xmallocz wrapper.c:83:9
    #4 0x9ab2d7 in xmemdupz wrapper.c:99:16
    #5 0x78d6a4 in common_prefix dir.c:191:15
    #6 0x5aca48 in cmd_ls_files builtin/ls-files.c:669:16
    #7 0x4cd92d in run_builtin git.c:453:11
    #8 0x4cb5fa in handle_builtin git.c:704:3
    #9 0x4ccf57 in run_argv git.c:771:4
    #10 0x4caf49 in cmd_main git.c:902:19
    #11 0x69ce2e in main common-main.c:52:11
    #12 0x7f64d4d94349 in __libc_start_main (/lib64/libc.so.6+0x24349)

Signed-off-by: Andrzej Hunt <ajrhunt@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>