We convert several output messages to use a spelling of
"de-duplication" consistent with all other messages from
the "git lfs dedup" command.
We also revise one message's text to clarify that it refers
to a Git LFS object file.
Note that one of these messages is not yet passed as a
translation string, but we will address that issue in a
subsequent commit.
Add tests for both the bare and -t flavours of "git lfs dedup"
to confirm that having any registered Git LFS extensions cause
the command to exit with the expected warning message.