Previously patch would always expect the file that it was patching to
exist (even it were empty). If we know that the patch is creating a file
from nothing (i.e has a start line of '0'), then we treat a file that
doesn't exist as if it has no content lines.
Implement the patch '-p' / '--strip' option, which strips the given
number of leading components from filenames parsed in the patch header.
If not given this option defaults to the basename of that path.
This is still very bare bones, and there is _much_ more to still
handle. However, this implements enough functionality to parse a single
unified patch read from stdin, and apply it to a file.
