[RFC PATCH v2 0/7] Improve path collision conflict resolutions

This RFC series depends on this series submitted yesterday:
  https://public-inbox.org/git/20181012212551.7689-1-newren@gmail.com/
(which in turn depends on en/merge-cleanup in next).

Although this is an "update" to my previous RFC series from six months
ago:
  https://public-inbox.org/git/20180305171125.22331-1-newren@gmail.com/
It's essentially a complete rewrite to instead use the strategy proposed
by Jonathan and Junio (from a separate thread discussing that RFC series):
  https://public-inbox.org/git/20180312213521.GB58506@aiede.svl.corp.google.com/
  https://public-inbox.org/git/CAPc5daVu8vv9RdGON8JiXEO3ycDVqQ38ySzZc-cpo+AQcAKXjA@mail.gmail.com

The basic idea is to make the "file collision" conflict types all behave
like add/add.  These types are:
  * add/add
  * rename/add
  * rename/rename(2to1)
  * each rename/add piece of a rename/rename(1to2)/add[/add] conflict

Specific bits that make this RFC:
  * Depends on another series not even in pu yet.
  * In order to simplify review, I add a new common function for all
    these conflict types to call in one commit, and since the function
    is not yet used at that point, I wrap the function in an #ifdef 0.
    In later patches, I remove the #ifdef.  Is that...okay?
  * There are a few FIXMEs in the code.  I'm okay leaving them there,
    but are others upset by them?
  * Every time I attempt to complete this series, I spot more problems;
    many of the merge-recursive series I've submitted since March were
    issues I discovered while trying to complete this series -- or
    secondary tangents discovered while working on the side-issues to
    this one.  I've gotten to the point of automatically second-guessing
    myself on whether I could have actually completed the necessary
    fixes.

Elijah Newren (7):
  Add testcases for consistency in file collision conflict handling
  t6036, t6042: testcases for rename collision of already conflicting
    files
  merge-recursive: new function for better colliding conflict
    resolutions
  merge-recursive: fix rename/add conflict handling
  merge-recursive: improve handling for rename/rename(2to1) conflicts
  merge-recursive: use handle_file_collision for add/add conflicts
  merge-recursive: improve rename/rename(1to2)/add[/add] handling

 merge-recursive.c                    | 513 ++++++++++++++++-----------
 t/t6036-recursive-corner-cases.sh    | 228 +++++++++++-
 t/t6042-merge-rename-corner-cases.sh | 333 ++++++++++++++++-
 t/t6043-merge-rename-directories.sh  | 107 +++---
 4 files changed, 893 insertions(+), 288 deletions(-)