Abort on empty username so specifying a username can be forced.

This behaviour was introduced by c3345b0f2fcd and broken by 98f2507c5551.
Added test for this.

mercurial/ui.py

@@ -345,12 +345,14 b' class ui(object):'
             user = self.config("ui", "username")
         if user is None:
             user = os.environ.get("EMAIL")
-        if not user:
+        if user is None:
             try:
                 user = '%s@%s' % (util.getuser(), socket.getfqdn())
+                self.warn(_("No username found, using '%s' instead\n") % user)
             except KeyError:
-                raise util.Abort(_("Please specify a username."))
-            self.warn(_("No username found, using '%s' instead\n") % user)
+                pass
+        if not user:
+            raise util.Abort(_("Please specify a username."))
         return user
 
     def shortuser(self, user):

tests/test-committer

@@ -24,5 +24,8 b' echo 1 > asdf'
 hg commit -d '1000000 0' -u "[e-mail]" -m commit-1
 hg tip
 echo 123 > asdf
+echo "[ui]" > .hg/hgrc
+echo "username = " >> .hg/hgrc
+hg commit -d '1000000 0' -m commit-1
 rm .hg/hgrc
 hg commit -d '1000000 0' -m commit-1 2>&1 | sed -e "s/'[^']*'/user@host/"

tests/test-committer.out

@@ -22,4 +22,7 b' user: [e-mail]'
 date:        Mon Jan 12 13:46:40 1970 +0000
 summary:     commit-1
 
+abort: Please specify a username.
+transaction abort!
+rollback completed
 No username found, using user@host instead