tests/test-revert-flags.t

copies: add a config to limit the number of candidates to check in heuristics

The heuristics algorithm find possible candidates for move/copy and then check
whether they are actually a copy or move. In some cases, there can be lot of
candidates possible which can actually slow down the algorithm.

This patch introduces a config option
`experimental.copytrace.movecandidateslimit` using which one can limit the
candidates to check. The limit defaults to 100.

Thanks to Yuya for suggesting to skip copytracing for that file with a
warning.

Differential Revision: https://phab.mercurial-scm.org/D987

#require execbit

  $ hg init repo
  $ cd repo
  $ echo foo > foo
  $ chmod 644 foo
  $ hg ci -qAm '644'

  $ chmod 755 foo
  $ hg ci -qAm '755'

reverting to rev 0

  $ hg revert -a -r 0
  reverting foo
  $ hg st
  M foo
  $ hg diff --git
  diff --git a/foo b/foo
  old mode 100755
  new mode 100644

  $ cd ..