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.
% file replaced with directory
adding a
% should fail - would corrupt dirstate
abort: file 'a' in dirstate clashes with 'a/a'
% directory replaced with file
adding a/a
% should fail - would corrupt dirstate
abort: directory 'a' already in dirstate
% directory replaced with file
adding b/c/d
% should fail - would corrupt dirstate
abort: directory 'b' already in dirstate