apply: handle Subversion diffs with /dev/null gracefully
Subversion generates diffs that can contain lines like this one:
--- /dev/null (nonexistent)
Let's teach Git's apply machinery to handle such a line gracefully.
This fixes https://github.com/git-for-windows/git/isues/1489
Signed-off-by: Tatyana Krasnukha <tatyana@synopsys.com> Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>