Argument parser seems to grab prefix

On line https://github.com/dolthub/dolt/blob/be862d796c7be139d73136e1b10515621086778d/go/cmd/dolt/commands/diff.go#L148
Adding the abbreviation `"m"` to the argument `DiffMode` causes `MergeBase` to become an invalid argument.

This changes the line from
```
ap.SupportsString(DiffMode, "", "diff mode", "Determines how to display modified rows with tabular output...
```
to
```
ap.SupportsString(DiffMode, "m", "diff mode", "Determines how to display modified rows with tabular output...
```
It seems like the `"m"` is matching against the first `"m"` in `"merge-base"`, which should not be happening. In either case, this is not the expected behavior, and should be fixed.