rev-list: use hdr_termination instead of a always using a newline

rev-list: use hdr_termination instead of a always using a newline

When adding support for prefixing output of log and other commands using
--line-prefix, commit 660e113ce118 ("graph: add support for
--line-prefix on all graph-aware output", 2016-08-31) accidentally
broke rev-list --header output.

In order to make the output appear with a line-prefix, the flow was
changed to always use the graph subsystem for display. Unfortunately
the graph flow in rev-list did not use info->hdr_termination as it was
assumed that graph output would never need to putput NULs.

Since we now always use the graph code in order to handle the case of
line-prefix, simply replace putchar('\n') with
putchar(info->hdr_termination) which will correct this issue.

Add a test for the --header case to make sure we don't break it in the
future.

Reported-by: Dennis Kaarsemaker <dennis@kaarsemaker.net>
Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>