cmdserver: mask return code of runcommand in the same way as dispatch.run
"hg help" does not state that the code for abort is 255, but it's confusing
to have different code between hg command and command server.
Tests of python-hglib 1.2 passed with this change.
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown