clone: abort if default destination has no meaningful name (BC)

If source URL has no path, default destination is resolved as '.'. It is
surprising than useful, and perhaps an unexpected behavior.

This change does not solve issue3880, but can avoid to clone into current
directory by accident.

mercurial/hg.py

@@ -132,13 +132,16 b' def defaultdest(source):'
     >>> defaultdest('/')
     ''
     >>> defaultdest('')
-    '.'
+    ''
     >>> defaultdest('http://example.org/')
-    '.'
+    ''
     >>> defaultdest('http://example.org/foo/')
     'foo'
     '''
-    return os.path.basename(os.path.normpath(util.url(source).path or ''))
+    path = util.url(source).path
+    if not path:
+        return ''
+    return os.path.basename(os.path.normpath(path))
 
 def share(ui, source, dest=None, update=True):
     '''create a shared repository'''
@@ -290,7 +293,8 b' def clone(ui, peeropts, source, dest=Non'
 
     if dest is None:
         dest = defaultdest(source)
-        ui.status(_("destination directory: %s\n") % dest)
+        if dest:
+            ui.status(_("destination directory: %s\n") % dest)
     else:
         dest = ui.expandpath(dest)
 

tests/test-http-clone-r.t

@@ -197,4 +197,11 b' clone remote via stream'
   checking files
   4 files, 9 changesets, 7 total revisions
   $ cd ..
+
+no default destination if url has no path:
+
+  $ hg clone http://localhost:$HGPORT/
+  abort: empty destination path is not valid
+  [255]
+
   $ cat error.log