MinGW: truncate exit()'s argument to lowest 8 bits

MinGW: truncate exit()'s argument to lowest 8 bits

For some reason, MinGW's bash cannot reliably detect failure of the child
process if a negative value is passed to exit(). This fixes it by
truncating the exit code in all calls of exit().

This issue was worked around in run_builtin() of git.c (2488df84 builtin
run_command: do not exit with -1, 2007-11-15). This workaround is no longer
necessary and is reverted.

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