gitlint: Add "commit" to violation messages

This PR clarifies the violation messages emitted by gitlint when
checking the commit message. For example:

 * Before: `43: UC4 Line exceeds max length (N>75): "..."`
 * After: `43: UC4 Commit line exceeds max length (N>75): "..."`

This makes it easier to identify the source of the error since there is
currently no additional context besides the error code UC*. I recently
pushed a commit that had some sample code as part of the commit body
that exceeded the lenght limit, and thought the error was referring to
one of my source files based on the line it showed. (feel free to laugh
at me, but let's make it better for the next person)

Signed-off-by: Tristan Honscheid <honscheid@google.com>