ui: check EOF of getpass() response read from command-server channel

readline() returns '' only when EOF is encountered, in which case, Python's
getpass() raises EOFError. We should do the same to abort the session as
"response expected."

This bug was reported to
https://bitbucket.org/tortoisehg/thg/issues/4659/

mercurial/ui.py

@@ -972,7 +972,10 b' class ui(object):'
             # disable getpass() only if explicitly specified. it's still valid
             # to interact with tty even if fin is not a tty.
             if self.configbool('ui', 'nontty'):
-                return self.fin.readline().rstrip('\n')
+                l = self.fin.readline()
+                if not l:
+                    raise EOFError
+                return l.rstrip('\n')
             else:
                 return getpass.getpass('')
         except EOFError:

tests/test-commandserver.t

@@ -607,6 +607,12 b' changelog and manifest would have invali'
   ...     runcommand(server, ['debuggetpass', '--config',
   ...                         'ui.interactive=True'],
   ...                input=stringio('1234\n'))
+  ...     runcommand(server, ['debuggetpass', '--config',
+  ...                         'ui.interactive=True'],
+  ...                input=stringio('\n'))
+  ...     runcommand(server, ['debuggetpass', '--config',
+  ...                         'ui.interactive=True'],
+  ...                input=stringio(''))
   ...     runcommand(server, ['debugprompt', '--config',
   ...                         'ui.interactive=True'],
   ...                input=stringio('5678\n'))
@@ -614,6 +620,11 b' changelog and manifest would have invali'
   ...     runcommand(server, ['debugwritestdout'])
   *** runcommand debuggetpass --config ui.interactive=True
   password: 1234
+  *** runcommand debuggetpass --config ui.interactive=True
+  password: 
+  *** runcommand debuggetpass --config ui.interactive=True
+  password: abort: response expected
+   [255]
   *** runcommand debugprompt --config ui.interactive=True
   prompt: 5678
   *** runcommand debugreadstdin