apply: compute patch->def_name correctly under -p0

apply: compute patch->def_name correctly under -p0

Back when "git apply" was written, we made sure that the user can
skip more than the default number of path components (i.e. 1) by
giving "-p<n>", but the logic for doing so was built around the
notion of "we skip N slashes and stop".  This obviously does not
work well when running under -p0 where we do not want to skip any,
but still want to skip SP/HT that separates the pathnames of
preimage and postimage and want to reject absolute pathnames.

Stop using "stop_at_slash()", and instead introduce a new helper
"skip_tree_prefix()" with similar logic but works correctly even for
the -p0 case.

This is an ancient bug, but has been masked for a long time because
most of the patches are text and have other clues to tell us the
name of the preimage and the postimage.

Noticed by Colin McCabe.

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