tests: don't rely on broken behaviour in test-largefiles-cache.t
The test relied on the bug that 'pull largefiles from branchheads' didn't pull
any largefiles from tip revision when it seemed like no largefiles had been
checked out before.
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$ hg init
$ echo '[Section]' >> $HGRCPATH
$ echo 'KeY = Case Sensitive' >> $HGRCPATH
$ echo 'key = lower case' >> $HGRCPATH
$ hg showconfig Section
Section.KeY=Case Sensitive
Section.key=lower case