Switch RepositoryCommitModel to use Commit.Message instead of Commit.Mes...

...sageShort so the complete text of the commit message will be available in the Commits and Commit views.

Note: Commit.Message includes embedded '\n' characters as-is; this change does not attempt to translate them to line-breaks (ex: via "BR/").

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Without something like this change, there doesn't seem to be a way to see the complete message of a commit.