merge: fix spurious merges for copies in linear updates
We make better use of contexts to accurately identify copies that
don't need merges.
Add a simple test and update other tests.
#!/bin/shecho 'syntax error' > badext.pyabspath=`pwd`/badext.pyecho '[extensions]' >> $HGRCPATHecho "badext = $abspath" >> $HGRCPATHhg -q help help