pager: do wait_for_pager on signal death

pager: do wait_for_pager on signal death

Since ea27a18 (spawn pager via run_command interface), the
original git process actually does git work, and the pager
is a child process (actually, on Windows it has always been
that way, since Windows lacks fork). After spawning the
pager, we register an atexit() handler that waits for the
pager to finish.

Unfortunately, that handler does not always run. In
particular, if git is killed by a signal, then we exit
immediately. The calling shell then thinks that git is done;
however, the pager is still trying to run and impact the
terminal. The result can be seen by running a long git
process with a pager (e.g., "git log -p") and hitting ^C.
Depending on your config, you should see the shell prompt,
but pressing a key causes the pager to do any terminal
de-initialization sequence.

This patch just intercepts any death-dealing signals and
waits for the pager before dying. Under typical less
configuration, that means hitting ^C will cause git to stop
generating output, but the pager will keep running.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>