Ask git for the attribute of every conflicted file whenever we load the
file status, so that we recognize the markers it actually wrote. Files
that are set up this way are precisely the ones whose regular content
tends to contain marker-looking lines, so matching a run of at least
seven characters instead is not an option: we'd take the file's own
content for markers and then never consider its conflicts resolved.
One `git check-attr` call covers all conflicted files at once; asking per
file would take seconds when hundreds of files are conflicted, and it
would hurt worst on Windows, where spawning a process is expensive.
Because the lookup rides along with the file status, it costs nothing
when there are no conflicts, and editing .gitattributes during a merge
takes effect on the next refresh.
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