tests/test-gendoc-zh_TW.t

revlog: finer computation of "issnapshot"

If the parent had an empty diff, we skip of it to compute a diff against the
parent base. This create shorter and simpler chain.

However these case could be wrongly detected as snapshot. So we improve the code
doing this detection.

In practice nobody care as when tried on a copy of mozilla-try and we got the
same number of snapshot (1315) in both case.

Performance where equivalent.

#require docutils gettext

  $ $TESTDIR/check-gendoc zh_TW
  checking for parse errors