Fix the filename-comparison code to actually work correctly.

Summary:
I was testing the current directory against the ignore-patterns in
babel.ini, but I was only looking at the last component of the current
directory, not the full directory-name relative to ka-root, like I'm
supposed to be doing.  Do that now.

Test Plan:
Ran 'make i18n' with some instrumentation here:

                if (os.path.basename(pattern) == '**'
                    and pathmatch(os.path.dirname(pattern), this_dirname)):
                    print 'IIIIII', this_dirname  # <--- INSTRUMENTATION
                    del dirnames[i]
                    break

I saw this output, which matches what's in babel.ini:

IIIIII tools
IIIIII third_party
IIIIII stylesheets
IIIIII images
IIIIII flvplayer
IIIIII deploy
IIIIII clienttemplates
IIIIII genfiles/translations
IIIIII genfiles/stylesheets
IIIIII genfiles/javascript
IIIIII genfiles/compressed_stylesheets
IIIIII genfiles/compressed_javascript
IIIIII genfiles/compiled_handlebars_templates
IIIIII genfiles/combined-stylesheets-prod
IIIIII javascript/test
IIIIII javascript/scratchpads-exec-package/tests
IIIIII khan-exercises/test
IIIIII khan-exercises/local-only
IIIIII khan-exercises/images
IIIIII khan-exercises/build
IIIIII khan-exercises/exercises/test
IIIIII khan-exercises/utils/test
IIIIII khan-exercises/utils/MathJax
IIIIII socrates/node_modules
IIIIII templates/intl

The 'make i18n_pot' step runs a lot faster now, too. :-)

Reviewers: chris, john

Reviewed By: john

Differential Revision: http://phabricator.khanacademy.org/D3787