scmutil: respect ui.relative-paths in default match.badfn

Authored by martinvonz.

Description

scmutil: respect ui.relative-paths in default match.badfn

We should probably be passing in a uipathfn here instead, so the
caller can get consistent output (between match.badfn and whatever
else it prints). I think we should ideally remove the badfn from the
matcher completely, but that's a different story. This patch is at
least not making it worse.

Differential Revision: https://phab.mercurial-scm.org/D5916

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