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[RFC PATCH 0/7] merge requirement: index matches head

Between working on some other things, I happened to be reading
Documentation/git-merge.txt and ran across the part that says:

    ...[merge will] abort if there are any changes registered in the
    index relative to the `HEAD` commit.  (One exception is when the
    changed index entries are in the state that would result from the
    merge already.)

I was pretty sure this statement was wrong, but did some digging to
uncover the details and the history.  What I thought would turn into a
simple three-line documentation fix, ballooned into this patch series.

This series might be best read in a different order; I'm not yet sure
the right way to structure it.  But:

  * Patch 5 demonstrates one of the ways that the parenthetical
    sentence is wrong (desirable perhaps, but not what is implemented)

  * Patch 7 explains the history, the trade-offs, the three ways the
    parenthetical sentence is wrong, and the many pitfalls we've run
    into trying to allow for such an exception.  Very small
    documentation fix with a huge commit message.

  * Patch 6 fixes the breakage demonstrated in patch 5, but if I only
    submitted patches 5-7, then the testsuite wouldn't pass because
    this fix uncovered multiple other bugs.  That's where patches 1-4
    came in.  This fix is also kind of opinionated; it takes the stance
    that allowing the exceptions isn't worth it.

Elijah Newren (7):
  t6044: verify that merges expected to abort actually abort
  t6044: add a testcase for index matching head, when head doesn't match HEAD
  merge-recursive: make sure when we say we abort that we actually abort
  merge-recursive: fix assumption that head tree being merged is HEAD
  t6044: add more testcases with staged changes before a merge is invoked
  merge-recursive: enforce rule that index matches head before merging
  merge: fix misleading pre-merge check documentation

 Documentation/git-merge.txt              |   6 +-
 builtin/am.c                             |   6 +-
 cache.h                                  |   8 --
 merge-recursive.c                        |  14 +--
 merge.c                                  |  10 +-
 t/t6044-merge-unrelated-index-changes.sh |  67 +++++++++++--
 t/t7504-commit-msg-hook.sh               |   4 +-
 t/t7611-merge-abort.sh                   | 118 -----------------------
 tree.h                                   |   8 ++
 9 files changed, 87 insertions(+), 154 deletions(-)