tests/test-revlog-group-emptyiter.t

merge: use operation-provided labels (ex: dest/source) in several merge-tools

Tools that did not use labels already, used only one, or used some label other
than the exact strings of "local" or "other" were unmodified.  Cases that used
the label "base" were modified as well, if they were otherwise changed in this
CL; "merged" was *not* changed.

There are other possible changes we might want to make, but I didn't:
- bcompare (linux and osx) uses the labels "parent1" and "parent2" instead of
  "local" and "other", so it was left alone, even though beyondcompare3
  (windows) *was* changed.
- araxis used the labels "Other", "Base", and "Local :$local", so it was also
  left alone.
- UltraCompare didn't provide a label for 'local', just for 'base' and 'other',
  so it was left alone.

Differential Revision: https://phab.mercurial-scm.org/D2012

Issue1678: IndexError when pushing

setting up base repo
  $ hg init a
  $ cd a
  $ touch a
  $ hg ci -Am a
  adding a
  $ cd ..

cloning base repo
  $ hg clone a b
  updating to branch default
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ cd b

setting up cset to push
  $ hg up null
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ touch a
different msg so we get a clog new entry
  $ hg ci -Am b
  adding a
  created new head

pushing
  $ hg push -f ../a
  pushing to ../a
  searching for changes
  adding changesets
  adding manifests
  adding file changes
  added 1 changesets with 0 changes to 0 files (+1 heads)

  $ cd ..