patch-id: ignore newline at end of file in diff_flush_patch_id()

patch-id: ignore newline at end of file in diff_flush_patch_id()

Whitespace is ignored when calculating patch IDs.  This is done by
removing all whitespace from diff lines before hashing them, including
a newline at the end of a file.  If that newline is missing, however,
diff reports that fact in a separate line containing "\ No newline at
end of file\n", and this marker is hashed like a context line.

This goes against our goal of making patch IDs independent of
whitespace.  Use the same heuristic that 2485eab55cc (git-patch-id: do
not trip over "no newline" markers, 2011-02-17) added to git patch-id
instead and skip diff lines that start with a backslash and a space
and are longer than twelve characters.

Reported-by: Tilman Vogel <tilman.vogel@web.de>
Initial-test-by: Tilman Vogel <tilman.vogel@web.de>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>