The patch structure has def_name component that is used to validate the
sanity of a "diff --git" patch by checking pathnames that appear on the
patch header lines for consistency.  The git_header_name() function is
used to compute this out of "diff --git a/... b/..." line, but the code
always stripped one level of prefix (i.e. "a/" and "b/"), without paying
attention to -p<n> option.  Code in find_name() function that parses other
lines in the patch header (e.g. "--- a/..." and "+++ b/..." lines) however
did strip the correct number of leading paths prefixes, and the sanity
check between these computed values failed.

Teach git_header_name() to honor -p<n> option like find_name() function
does.

Found and reported by Steven J. Murdoch who also wrote tests.

Signed-off-by: Junio C Hamano <gitster@pobox.com>