CheckLog: show which lines are bigger than the limit

The error message is more useful if it tells which lines are bigger and
not just the number of lines that are.
CheckLog: Fix detection of big log body lines

The check was wrong by one. It considered a line with exactly the
body-max-width as too big.
CheckFile: Implement this new plugin

While CheckStructure checks the names of files and references, CheckFile
checks the file's contents.
v0.052

  [New Features]

  - New plugin Git::Hooks::CheckFile checks the contents of files with
    external commands. Which command is used to check a file is based on the
    filename.

  [Changes]

  - Drop configuration backwards compatibility. Up to version 0.31, as of
    January 2013, there were some changes in the way Git::Hooks and its
    plugins were configured. Up to version 0.051 those old ways were still
    supported. This version drops that compatibility baggage.

  - CheckLog now shows which lines are bigger than body-max-width in the
    error message.

  [Fixes]

  - The output of external hooks that fail are now sent to Gerrit as a
    comment.

  - CheckLog was complaining about lines with exactly body-max-width
    characters. Now it lets them pass.