fmt-merge-msg: free newly allocated temporary strings when done

fmt-merge-msg: free newly allocated temporary strings when done

origin starts off pointing to somewhere within line, which is owned by
the caller. Later we might allocate a new string using xmemdupz() or
xstrfmt(). To avoid leaking these new strings, we introduce a to_free
pointer - which allows us to safely free the newly allocated string when
we're done (we cannot just free origin directly as it might still be
pointing to line).

LSAN output from t0090:

Direct leak of 8 byte(s) in 1 object(s) allocated from:
    #0 0x49a82d in malloc ../projects/compiler-rt/lib/asan/asan_malloc_linux.cpp:145:3
    #1 0xa71f49 in do_xmalloc wrapper.c:41:8
    #2 0xa720b0 in do_xmallocz wrapper.c:75:8
    #3 0xa720b0 in xmallocz wrapper.c:83:9
    #4 0xa720b0 in xmemdupz wrapper.c:99:16
    #5 0x8092ba in handle_line fmt-merge-msg.c:187:23
    #6 0x8092ba in fmt_merge_msg fmt-merge-msg.c:666:7
    #7 0x5ce2e6 in prepare_merge_message builtin/merge.c:1119:2
    #8 0x5ce2e6 in collect_parents builtin/merge.c:1215:3
    #9 0x5c9c1e in cmd_merge builtin/merge.c:1454:16
    #10 0x4ce83e in run_builtin git.c:475:11
    #11 0x4ccafe in handle_builtin git.c:729:3
    #12 0x4cb01c in run_argv git.c:818:4
    #13 0x4cb01c in cmd_main git.c:949:19
    #14 0x6b3fad in main common-main.c:52:11
    #15 0x7fb929620349 in __libc_start_main (/lib64/libc.so.6+0x24349)

SUMMARY: AddressSanitizer: 8 byte(s) leaked in 1 allocation(s).

Signed-off-by: Andrzej Hunt <andrzej@ahunt.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>