[merge][1.6][#234748] fix KnitCorrupt error in check on bzr.dev

[Martin Pool]

Happily to say the repository is not corrupt (and this is an
increasingly poorly-named error) but there is a bug in the knit code
for extracting several values at a time.

The basic problem is of memory aliasing; we have a dict pointing to
the contents of other file text and accidentally mutate a list within
it.  The previous code tried to protect against this but failed.  The
main point is that calling .text() on a KnitContent object will
already take trailing eols into account so no special handling is
needed here.

I'd kind of like to remove more of the special-casing here of
extraction of multiple files, but I guess it is there as an
optimization so shouldn't be done without measurement.

Thanks very much to spiv for tracking this down with me.

This still needs a test and news entry but I thought I'd put up the
core fix now.

-- 
Martin <http://launchpad.net/~mbp/>
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