Implement GraphQLApplier (#15)

GitHub recently added the createCommitOnBranch mutation, which
simplifies commit creation and adds free signing support, with some
limitations that don't make it obviously better than the old approach in
all cases.

Due to these downsides, the different contract, and to support GitHub
Enterprise, implement this as a new type instead of modifying the
existing applier.

Expand testing to cover the new applier, including patches that it does
not support. Add the new IsUnsupported function to test if apply errors
are due to GraphQL limitations. There's some possibly inefficient code
that gets file modes to check for these limitations, but I don't think
it's worth optimizing at this point.

Author: bluekeyes

applier_test.go

@@ -15,6 +15,7 @@ import (
 
 	"github.com/bluekeyes/go-gitdiff/gitdiff"
 	"github.com/google/go-github/v39/github"
+	"github.com/shurcooL/githubv4"
 	"golang.org/x/oauth2"
 )
 
@@ -61,7 +62,8 @@ type TestContext struct {
 	BaseCommit *github.Commit
 	BaseTree   *github.Tree
 
-	Client *github.Client
+	Client   *github.Client
+	V4Client *githubv4.Client
 }
 
 func (tctx *TestContext) Branch(name string) string {
@@ -234,15 +236,16 @@ func prepareTestContext(t *testing.T) *TestContext {
 	}
 
 	ctx := context.Background()
-	client := github.NewClient(oauth2.NewClient(ctx, oauth2.StaticTokenSource(
+	httpClient := oauth2.NewClient(ctx, oauth2.StaticTokenSource(
 		&oauth2.Token{AccessToken: token},
-	)))
+	))
 
 	tctx := TestContext{
-		Context: ctx,
-		ID:      id,
-		Repo:    repo,
-		Client:  client,
+		Context:  ctx,
+		ID:       id,
+		Repo:     repo,
+		Client:   github.NewClient(httpClient),
+		V4Client: githubv4.NewClient(httpClient),
 	}
 	return &tctx
 }

error.go

@@ -0,0 +1,38 @@
+package patch2pr
+
+import (
+	"errors"
+	"fmt"
+)
+
+func unsupported(msg string, args ...interface{}) error {
+	return unsupportedErr{reason: fmt.Sprintf(msg, args...)}
+}
+
+type unsupportedErr struct {
+	reason string
+}
+
+func (err unsupportedErr) Error() string {
+	return fmt.Sprintf("unsupported: %s", err.reason)
+}
+
+func (err unsupportedErr) Unsupported() bool {
+	return true
+}
+
+// IsUnsupported returns true if err is the result of trying an unsupported
+// operation. It is equivalent to finding the first error in err's chain that
+// implements
+//
+//     type unsupported interface {
+//         Unsupported() bool
+//     }
+//
+// and then calling the Unsupported() method.
+func IsUnsupported(err error) bool {
+	var u interface {
+		Unsupported() bool
+	}
+	return errors.As(err, &u) && u.Unsupported()
+}

go.mod

@@ -5,5 +5,7 @@ go 1.14
 require (
 	github.com/bluekeyes/go-gitdiff v0.6.0
 	github.com/google/go-github/v39 v39.0.0
+	github.com/shurcooL/githubv4 v0.0.0-20210922025249-6831e00d857f
+	github.com/shurcooL/graphql v0.0.0-20200928012149-18c5c3165e3a // indirect
 	golang.org/x/oauth2 v0.0.0-20200107190931-bf48bf16ab8d
 )

go.sum

@@ -12,6 +12,10 @@ github.com/google/go-github/v39 v39.0.0 h1:pygGA5ySwxEez1N39GnDauD0PaWWuGgayudyZ
 github.com/google/go-github/v39 v39.0.0/go.mod h1:C1s8C5aCC9L+JXIYpJM5GYytdX52vC1bLvHEF1IhBrE=
 github.com/google/go-querystring v1.1.0 h1:AnCroh3fv4ZBgVIf1Iwtovgjaw/GiKJo8M8yD/fhyJ8=
 github.com/google/go-querystring v1.1.0/go.mod h1:Kcdr2DB4koayq7X8pmAG4sNG59So17icRSOU623lUBU=
+github.com/shurcooL/githubv4 v0.0.0-20210922025249-6831e00d857f h1:q4b8/GCL8Ksl+okhFKSd8DVBQNc0XExjxTO68nK0c3Y=
+github.com/shurcooL/githubv4 v0.0.0-20210922025249-6831e00d857f/go.mod h1:hAF0iLZy4td2EX+/8Tw+4nodhlMrwN3HupfaXj3zkGo=
+github.com/shurcooL/graphql v0.0.0-20200928012149-18c5c3165e3a h1:KikTa6HtAK8cS1qjvUvvq4QO21QnwC+EfvB+OAuZ/ZU=
+github.com/shurcooL/graphql v0.0.0-20200928012149-18c5c3165e3a/go.mod h1:AuYgA5Kyo4c7HfUmvRGs/6rGlMMV/6B1bVnB9JxJEEg=
 golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACkg1iLfiJU5Ep61QUkGW8qpdssI0+w=
 golang.org/x/crypto v0.0.0-20210817164053-32db794688a5 h1:HWj/xjIHfjYU5nVXpTM0s39J9CbLn7Cc5a7IC5rwsMQ=
 golang.org/x/crypto v0.0.0-20210817164053-32db794688a5/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=