merge-ort: replace string_list_df_name_compare with faster alternative

merge-ort: replace string_list_df_name_compare with faster alternative

Gathering accumulated times from trace2 output on the mega-renames
testcase, I saw the following timings (where I'm only showing a few
lines to highlight the portions of interest):

    10.120 : label:incore_nonrecursive
        4.462 : ..label:process_entries
           3.143 : ....label:process_entries setup
              2.988 : ......label:plist special sort
           1.305 : ....label:processing
        2.604 : ..label:collect_merge_info
        2.018 : ..label:merge_start
        1.018 : ..label:renames

In the above output, note that the 4.462 seconds for process_entries was
split as 3.143 seconds for "process_entries setup" and 1.305 seconds for
"processing" (and a little time for other stuff removed from the
highlight).  Most of the "process_entries setup" time was spent on
"plist special sort" which corresponds to the following code:

    trace2_region_enter("merge", "plist special sort", opt->repo);
    plist.cmp = string_list_df_name_compare;
    string_list_sort(&plist);
    trace2_region_leave("merge", "plist special sort", opt->repo);

In other words, in a merge strategy that would be invoked by passing
"-sort" to either rebase or merge, sorting an array takes more time than
anything else.  Serves me right for naming my merge strategy this way.

Rewrite the comparison function in a way that does not require finding
out the lengths of the strings when comparing them.  While at it, tweak
the code for our specific case -- no need to handle a variety of modes,
for example.  The combination of these changes reduced the time spent in
"plist special sort" by ~25% in the mega-renames case.

For the testcases mentioned in commit 557ac0350d ("merge-ort: begin
performance work; instrument with trace2_region_* calls", 2020-10-28),
this change improves the performance as follows:

                            Before                  After
    no-renames:        5.622 s ±  0.059 s     5.235 s ±  0.042 s
    mega-renames:     10.127 s ±  0.073 s     9.419 s ±  0.107 s
    just-one-mega:   500.3  ms ±  3.8  ms   480.1  ms ±  3.9  ms

Signed-off-by: Elijah Newren <newren@gmail.com>
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>