tests/test-diff-issue2761.t

lfs: improve the hints for common errors in the Batch API

The previous message was too debug-ish and less action oriented than a hint
should be.  The remaining errors that aren't handled are more along the lines of
programming errors (not using POST, bad accept type, etc), so I'm not bothering
with that.

The friendly errors purposely use `self.baseurl` instead of the full Batch API
endpoint because I'd expect some copy/paste/modify on the part of the user here,
and it would be more confusing if '/objects/batch' magically appeared, but
shouldn't be used in the config setting.  It still seems like the right thing
for debugging in the catchall case.

Test issue2761

  $ hg init

  $ touch to-be-deleted
  $ hg add
  adding to-be-deleted
  $ hg ci -m first
  $ echo a > to-be-deleted
  $ hg ci -m second
  $ rm to-be-deleted
  $ hg diff -r 0

Same issue, different code path

  $ hg up -C
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ touch does-not-exist-in-1
  $ hg add
  adding does-not-exist-in-1
  $ hg ci -m third
  $ rm does-not-exist-in-1
  $ hg diff -r 1