diff: refactor output of incomplete line

diff: refactor output of incomplete line

Create a helper function that reacts to "\ No newline at the end of
file" in preparation for unifying the incomplete line handling in
the code path that handles xdiff output and the code path that
bypasses xdiff and produces a complete-rewrite patch.

Currently the output from the DIFF_SYMBOL_CONTEXT_INCOMPLETE case
still (ab)uses the same code as what is used for context lines, but
that would change in a later step where we introduce support to treat
an incomplete line as a whitespace error.

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