diff -r e84438846c27 -r 4f3e989536c3 mercurial/scmutil.py
--- a/mercurial/scmutil.py	Tue Feb 13 00:05:45 2018 +0100
+++ b/mercurial/scmutil.py	Mon Feb 12 16:09:31 2018 +0100
@@ -267,6 +267,8 @@
         raise error.Abort(_("cannot use an integer as a name"))
     except ValueError:
         pass
+    if lbl.strip() != lbl:
+        raise error.Abort(_("leading or trailing whitespace in name %r") % lbl)
 
 def checkfilename(f):
     '''Check that the filename f is an acceptable filename for a tracked file'''