Add a keybinding to vc-dir

[Chong Yidong]

In the VC log, the "d" key calls log-view-diff.
In VC-dir, the "=" key calls vc-diff (to match C-x v =).

For mnemonic purposes, it might be good if log-view mode also binds "="
to log-view-diff, and VC-dir binds "d" to vc-diff.

Any objections?  The only obvious problem with doing this is if you (or
others) plan to introduce more operations to vc-dir in the future, so
that we need to conserve scarce keys.  That doesn't seem like an issue
to me, but thought I'd check in first.