apply: fix new-style empty context line triggering incomplete-line check

apply: fix new-style empty context line triggering incomplete-line check

A new-style unified context diff represents an empty context line
with an empty line (instead of a line with a single SP on it).  The
code to check whitespace errors in an incoming patch is designed to
omit the first byte of a line (typically SP, "-", or "+") and pass the
remainder of the line to the whitespace checker.

Usually we do not pass a context line to the whitespace error checker,
but when we are correcting errors, we do.  This "remove the first
byte and send the remainder" strategy of checking a line ended up
sending a zero-length string to the whitespace checker when seeing a
new-style empty context line, which caused the whitespace checker to
say "ah, you do not even have a newline at the end!", leading to an
"incomplete line" in the middle of the patch!

Fix this by pretending that we got a traditional empty context line
when we drive the whitespace checker.

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