Use ssh-askpass implementation from xenocara.

Index: cola/app.py
--- cola/app.py.orig
+++ cola/app.py
@@ -109,7 +109,7 @@ def setup_environment():
     elif sys.platform == 'darwin':
         askpass = resources.package_command('ssh-askpass-darwin')
     else:
-        askpass = resources.package_command('ssh-askpass')
+        askpass = '${X11BASE}/bin/ssh-askpass'
 
     compat.setenv('GIT_ASKPASS', askpass)
     compat.setenv('SSH_ASKPASS', askpass)