This change implements the get_file_content and get_file_size methods

for the Mercurial client backend.
Additionally, this changes the log methods to always use {node}

instead of {node|short}. We had a mix (individual commits were using

the full node SHA, while the cumulative diff was using the short form),

which was causing issues with file uploads--we'd upload a file for the

commit version with the full SHA revision, and then not find it because

the cumulative diff was using the short SHA.

Ran unit tests.
Uploaded a variety of revisions including binary files. Saw that

  everything worked correctly.