progress: treat "no terminal" as being in the foreground

Commit 85cb8906 (progress: no progress in background,
2015-04-13) avoids sending progress from background
processes by checking that the process group id of the
current process is the same as that of the controlling
terminal.

If we don't have a terminal, however, this check never
succeeds, and we print no progress at all (until the final
"done" message). This can be seen when cloning a large
repository; instead of getting progress updates for
"counting objects", it will appear to hang then print the
final count.

We can fix this by treating an error return from tcgetpgrp()
as a signal to show the progress.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>