tests/helper-runtests.sh

convert: don't drop missing or corrupt tag entries

Cleaning up the tags file could be a useful feature in some cases, so maybe
there should be a switch for this.  However, the default hg -> hg convert tries
to maintain identical hashes (thus convert.hg.saverev is off by default, but is
on by default for other source types).  It looks like _rewritesubstate() has a
`continue` in it, and therefore a similar problem.

I ran into this conversion divergence when a coworker "merged" two repositories
by copy/pasting all of the files from the source repo and massaging the code,
and forgetting to revert the .hg* files.  That silently emptied the .hgtags file
after the conversion.  (This isn't the manifest node bug Yuya has been helping
with- this occurred well after the bzr -> hg conversion and wasn't a merge
commit, which made it extra puzzling.  That bug is still an issue.)

#
# Avoid interference from actual test env:

unset HGTEST_JOBS
unset HGTEST_TIMEOUT
unset HGTEST_PORT
unset HGTEST_SHELL