[PATCH 0/1] And so it begins...merge/rename performance work

This depends on a merge of en/ort-conflict-handling, en/diffcore-rename, and en/ort-directory-rename.

This series begins the performance work for merge-ort and diffcore-rename. This series only has one patch and all it does is add trace2_region enter/leave pairs -- but it comes with a lengthy commit message detailing my driving testcases, the current status, and my plans. Part of the point of the lengthy testcase description is it will allow me to repeatedly refer to it in subsequent series' commit messages with a paragraph of the form:

    For the testcases mentioned in commit 9542932eee ("merge-ort: begin
    performance work; instrument with trace2_region_* calls", 2020-10-28),
    this change improves the performance as follows:
    
                                  Before                  After
          no-renames:       12.975 s ±  0.037 s    12.904 s ±  0.069 s
          mega-renames:   5154.338 s ± 19.139 s  1670.582 s ±  0.904 s
          just-one-mega:   146.703 s ±  0.852 s    48.149 s ±  0.306 s

Elijah Newren (1):
  merge-ort: begin performance work; instrument with trace2_region_*
    calls

 diffcore-rename.c |  8 +++++++
 merge-ort.c       | 57 +++++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 65 insertions(+)