ui: restrict length of autogenerated blocked tags

Long autogenerated blocked tags tend to be because the command has an absolute
path; at Facebook, we've had a few where the tag is thousands of characters
long (in association with the mergedriver).

Change the default to use a suffix of a command as the default tag, limiting us
to 85 characters (for a 100 character tag). This is long enough to overflow a
standard terminal (thus be obviously autogenerated), but short enough to be
readable.

mercurial/ui.py

@@ -1235,7 +1235,10 b' class ui(object):'
         object as exception.
         '''
         if blockedtag is None:
-            blockedtag = 'unknown_system_' + cmd.translate(None, _keepalnum)
+            # Long cmds tend to be because of an absolute path on cmd. Keep
+            # the tail end instead
+            cmdsuffix = cmd.translate(None, _keepalnum)[-85:]
+            blockedtag = 'unknown_system_' + cmdsuffix
         out = self.fout
         if any(s[1] for s in self._bufferstates):
             out = self