dispatch: mask negative exit code recorded in blackbox log

That's what we do for the exit code delivered to the environment.

mercurial/dispatch.py

@@ -239,7 +239,7 b' def dispatch(req):'
             req.ui.log('uiblocked', 'ui blocked ms',
                        **pycompat.strkwargs(req.ui._blockedtimes))
         req.ui.log("commandfinish", "%s exited %d after %0.2f seconds\n",
-                   msg, ret, duration)
+                   msg, ret & 255, duration)
         try:
             req._runexithandlers()
         except: # exiting, so no re-raises

tests/test-blackbox.t

@@ -7,6 +7,9 b' setup'
   > @command(b'crash', [], b'hg crash')
   > def crash(ui, *args, **kwargs):
   >     raise Exception("oops")
+  > @command(b'abort', [], b'hg abort')
+  > def abort(ui, *args, **kwargs):
+  >     raise error.Abort(b"oops")
   > EOF
   $ abspath=`pwd`/myextension.py
 
@@ -44,6 +47,14 b' failure exit code'
   1970/01/01 00:00:00 bob @0000000000000000000000000000000000000000 (5000)> add non-existent exited 1 after * seconds (glob)
   1970/01/01 00:00:00 bob @0000000000000000000000000000000000000000 (5000)> blackbox
 
+abort exit code
+  $ rm ./.hg/blackbox.log
+  $ hg abort 2> /dev/null
+  [255]
+  $ hg blackbox -l 2
+  1970/01/01 00:00:00 bob @0000000000000000000000000000000000000000 (5000)> abort exited 255 after * seconds (glob)
+  1970/01/01 00:00:00 bob @0000000000000000000000000000000000000000 (5000)> blackbox -l 2
+
 unhandled exception
   $ rm ./.hg/blackbox.log
   $ hg crash 2> /dev/null