i18n: mark help strings for translation
The gettext function is just another name for the normal _ function
and it is used for translating docstrings when using _ would make
pygettext.py output a warning.
% should fail
abort: path contains illegal component: .hg/00changelog.i
adding a/a
% should fail
abort: path 'b/b' traverses symbolic link 'b'
% should succeed
% should still fail - maybe
abort: path 'b/b' traverses symbolic link 'b'