update: add tests for untracked local file

We don't seem to have any tests for updating to another revision when
there are untracked files on the local side that conflict with the
those on the remote side, so let's add tests. This shows how we
overwrite untracked ignored files when updating to a revision that
tracks the file.

--HG--
branch : stable
extra : rebase_source : 51628254c172765ba878b2cd4a6ea95b6959be6b
extra : histedit_source : 45124ff444a15d4ad25e6e765694275982463742
update: don't overwrite untracked ignored files on update

When looking for untracked files that would conflict with a tracked
file in the target revision (or the remote side of a merge), we
explcitly exclude ignored files. The code was added in 7e30f5f2285f
(merge: refactor unknown file conflict checking, 2012-02-09), but it
seems like only unknown, not ignored, files were considered since the
beginning of time.

Although ignored files are mostly build outputs and backup files, we
should still not overwrite them. Fix by simply removing the explicit
check.

--HG--
branch : stable
extra : rebase_source : 82730e6a5b4ac4f981521fad89c4c7707f2a551a
extra : amend_source : 63fd0b9c0ccd47e98c1869ccf459e8b91c9f3768
extra : histedit_source : 8abaa8c33349ac265c1e09de109add58449d4c85