builtin-apply.c: mark common context lines in lineinfo structure.

builtin-apply.c: mark common context lines in lineinfo structure.

This updates the way preimage and postimage in a patch hunk is
parsed and prepared for applying.  By looking at image->line[n].flag,
the code can tell if it is a common context line that is the
same between the preimage and the postimage.

This matters when we actually start applying a patch with
contexts that have whitespace breakages that have already been
fixed in the target file.