apply --whitespace=warn/error: diagnose blank at EOF

apply --whitespace=warn/error: diagnose blank at EOF

"git apply" strips new blank lines at EOF under --whitespace=fix option,
but neigher --whitespace=warn nor --whitespace=error paid any attention to
these errors.

Introduce a new whitespace error class, blank-at-eof, to make the
whitespace error handling more consistent.

The patch adds a new "linenr" field to the struct fragment in order to
record which line the hunk started in the input file, but this is needed
solely for reporting purposes.  The detection of this class of whitespace
errors cannot be done while parsing a patch like we do for all the other
classes of whitespace errors.  It instead has to wait until we find where
to apply the hunk, but at that point, we do not have an access to the
original line number in the input file anymore, hence the new field.

Depending on your point of view, this may be a bugfix that makes warn and
error in line with fix.  Or you could call it a new feature.  The line
between them is somewhat fuzzy in this case.

Strictly speaking, triggering more errors than before is a change in
behaviour that is not backward compatible, even though the reason for the
change is because the code was not checking for an error that it should
have.  People who do not want added blank lines at EOF to trigger an error
can disable the new error class.

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