bug#60569: 29.0.60; vc-pull-and-push unsupported on non-git vcs

[Juri Linkov]

Isn't it so that as a general rule we don't add a vc command
when it's not supported by more than 1 backend?

It seems fine to add a git-specific 'vc-git-pull-and-push',
but why to add 'vc-pull-and-push' supported only by git?

It's more strange that the docstring of 'vc-pull-and-push' says:

  "It also signals an error in a Bazaar bound branch."

whereas in fact it's not implemented for Bazaar at all.