git allows a patch file to contain multiple deltas to the same file:
although it does not produce files in this format itself, this could
be the result of concatenating two different patch files that affected
the same file.

git apply behaves by applying this next delta to the existing postimage
of the file.  We should do the same.  If we have previously seen a file,
and produced a postimage for it, we will load that postimage and apply
the current delta to that.  If we have not, get the file from the
preimage.