ui: only use "user@host" as username in noninteractive mode

We regularly see people on IRC ask how they can correct commits they
accidentally made without having configured a username. This change
will make Mercurial abort when a commit is made without a username.

If Mercurial is run without a TTY (from a cronjob or similar), a
username is constructed as usual. Schematically the changes are as
follows:

With ui.askusername=False:

                  old         new
  interactive     user@host   abort
  noninteractive  user@host   user@host

With ui.askusername=True:

                  old         new
  interactive     prompt      prompt
  noninteractive  user@host   user@host

mercurial/ui.py

@@ -179,7 +179,7 b' class ui(object):'
             user = os.environ.get("EMAIL")
         if user is None and self.configbool("ui", "askusername"):
             user = self.prompt(_("enter a commit username:"), default=None)
-        if user is None:
+        if user is None and not self.interactive():
             try:
                 user = '%s@%s' % (util.getuser(), socket.getfqdn())
                 self.warn(_("No username found, using '%s' instead\n") % user)