dispatch: exit with 8-bit exit code
The exit code returned from a program to the shell is unsigned 8-bit, but
Mercurial would sometimes try to exit with negative numbers or None. sys.exit
on Unix will convert that to 8-bit exit codes, but on Windows negative values
showed up as 0.
The exit code is now explicitly converted to unsigned 8-bit.
tests: don't let time go back before 0 o'clock in bisect3.t
Windows agree that it was
Thu Jan 01 00:00:00 1970 +0000
but it also claims that it was
Thu Dec 31 23:59:59 1969 -0000