write_file_v(): do not leave incomplete line at the end

write_file_v(): do not leave incomplete line at the end

All existing callers to this function use it to produce a text file
or an empty file, and a new callsite that mimick them must end their
payload with a LF.  If they forget to do so, the resulting file will
end with an incomplete line.

Teach write_file_v() to complete the incomplete line, if exists, so
that the callers do not have to.

With this, the caller-side fix in builtin/am.c becomes unnecessary.

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