[sort-of-BUG] merge-resolve cannot resolve "content/mode" conflict

[Jeff King <peff@peff.net>]

Imagine a merge where one side changes the content of a path and the
other changes the mode. Here's a minimal reproduction:

  git init repo && cd repo &&

  echo base >file &&
  git add file &&
  git commit -m base &&

  echo changed >file &&
  git commit -am content &&

  git checkout -b side HEAD^
  chmod +x file &&
  git commit -am mode

If I merge that with merge-recursive, I get what you'd expect: mode
10755, and content "changed".

However, with merge-resolve, I get a conflict:

  $ git merge -s resolve master
  Trying really trivial in-index merge...
  error: Merge requires file-level merging
  Nope.
  Trying simple merge.
  Simple merge failed, trying Automatic merge.
  Auto-merging file
  ERROR: permissions conflict: 100644->100755,100644 in file
  fatal: merge program failed
  Automatic merge failed; fix conflicts and then commit the result.

I think this is only a half-bug, really. It's definitely a funny
situation, and it's not unreasonable for a merge driver to punt on a
funny situation rather than resolving it. But I would say:

  - it would probably be a nice improvement to resolve this as
    merge-recursive does

  - the "ERROR" message is silly and misleading; the permissions resolve
    just fine, it is only that the combination with the content-level
    change confuses the script (but the output does not mention that).

This is a leftover from my experiments with merge-resolve versus
merge-recursive last fall, which resulted in a few actual bug-fixes. I
looked into fixing this case, too, at that time. It seemed possible, but
a little more involved than you might think (because the logic is driven
by a bunch of case statements, and this adds a multiplicative layer to
the cases; we might need to resolve the permissions, and _then_ see if
the content can be resolved).

So I didn't actually come up with a patch, but I figured I'd write it up
here for posterity. And just didn't get around to it until now.

-Peff