diff.c: move line ending check into emit_hunk_header

diff.c: move line ending check into emit_hunk_header

The emit_hunk_header() function is responsible for assembling a
hunk header and calling emit_line() to send the hunk header
to the output file.  Its only caller fn_out_consume() needs
to prepare for a case where the function emits an incomplete
line and add the terminating LF.

Instead make sure emit_hunk_header() to always send a
completed line to emit_line().

Signed-off-by: Stefan Beller <redacted>
Signed-off-by: Junio C Hamano <redacted>