chgserver: include [extdiff] in confighash extdiff's uisetup will register new commands. If we do not include it in confighash, changes to [extdiff] will not get new commands registered. This patch adds extdiff to confighash and makes it possible for chg to pass test-extdiff.t.

chg: silently inherit server exit code If chgserver aborts during startup, for example, error.ParseError when parsing a config file, chg client probably just wants to exit with a same exit code without printing other unrelated text. This patch changes the text "cmdserver exited with status" from abortmsg to debugmsg and exits with a same exit code.

debugshell: use absolute_import