Add -pr for inspecting GitHub pull requests

[bobg]

New command-line flag -pr allows comparing the head of a GitHub pull-request branch with its base to produce a report like this one.

This PR also adds a new modver -pr check in this repo's GitHub Actions workflow.

It also fixes a problem in .gitignore, substantially refactors and adds test coverage in cmd/modver, and works around an apparent go-git bug that manifests in GitHub Actions.

[bobg]

Modver result

This report was generated by Modver,
a Go package and command that helps you obey semantic versioning rules in your Go module.

This PR requires (at least) an increase in your module's minor version number.

checking Compare
  in parameters of func(olders []*golang.org/x/tools/go/packages.Package, newers []*golang.org/x/tools/go/packages.Package) github.com/bobg/modver/v2.Result
    struct field TypeErrors was added to golang.org/x/tools/go/packages.Package
      Minor