run_command: encode deadly signal number in the return value

run_command: encode deadly signal number in the return value

We now write the signal number in the error message if the program
terminated by a signal. The negative return value is constructed such that
after truncation to 8 bits it looks like a POSIX shell's $?:

   $ echo 0000 | { git upload-pack .; echo $? >&2; } | :
   error: git-upload-pack died of signal 13
   141

Previously, the exit code was 255 instead of 141.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>