We had both `repo()` and `mut_repo()` on `Transaction` and I think it
was easy to get confused and think that the former returned a
`&ReadonlyRepo` but both of them actually return a reference to
`MutableRepo` (the latter obviously returns a mutable reference). I
hope that renaming to the more idiomatic `repo_mut()` will help
clarify.
We could instead have renamed them to `mut_repo()` and
`mut_repo_mut()` but that seemed unnecessarily long. It would better
match the `mut_repo` variables we typically use, though.
It seems better to have the caller pass the transaction description
when we finish the transaction than when we start it. That way we have
all the information we want to include more readily available.
Each instance of the enum represents a single command, so singular
`*Command` seems better. That also seems to match the examples in
clap's documentation.
