Add tests for using the pr Finder outside of repo

This includes an acceptance test for `gh pr view` and a unit test on
`TestFind`

acceptance/testdata/pr/pr-view-outside-repo.txtar

@@ -0,0 +1,28 @@
+# Use gh as a credential helper
+exec gh auth setup-git
+
+# Create a repository with a file so it has a default branch
+exec gh repo create $ORG/$SCRIPT_NAME-$RANDOM_STRING --add-readme --private
+
+# Defer repo cleanup
+defer gh repo delete --yes $ORG/$SCRIPT_NAME-$RANDOM_STRING
+
+# Clone the repo
+exec gh repo clone $ORG/$SCRIPT_NAME-$RANDOM_STRING
+
+# Prepare a branch to PR
+cd $SCRIPT_NAME-$RANDOM_STRING
+exec git checkout -b feature-branch
+exec git commit --allow-empty -m 'Empty Commit'
+exec git push -u origin feature-branch
+
+# Create the PR
+exec gh pr create --title 'Feature Title' --body 'Feature Body'
+stdout2env PR_URL
+
+# Change directories
+cd ..
+
+# View the PR
+exec gh pr view $PR_URL
+stdout 'Feature Title'

pkg/cmd/pr/shared/finder_test.go

@@ -211,6 +211,38 @@ func TestFind(t *testing.T) {
 			wantPR:   13,
 			wantRepo: "https://example.org/OWNER/REPO",
 		},
+		{
+			name: "PR URL argument and not in a local git repo",
+			args: args{
+				selector:   "https://example.org/OWNER/REPO/pull/13/files",
+				fields:     []string{"id", "number"},
+				baseRepoFn: nil,
+				branchFn: func() (string, error) {
+					return "", &git.GitError{
+						Stderr:   "fatal: not a git repository (or any of the parent directories): .git",
+						ExitCode: 128,
+					}
+				},
+				branchConfig: stubBranchConfig(git.BranchConfig{}, &git.GitError{
+					Stderr:   "fatal: not a git repository (or any of the parent directories): .git",
+					ExitCode: 128,
+				}),
+				pushDefault: stubPushDefault("simple", nil),
+				remotePushDefault: stubRemotePushDefault("", &git.GitError{
+					Stderr:   "fatal: not a git repository (or any of the parent directories): .git",
+					ExitCode: 128,
+				}),
+			},
+			httpStub: func(r *httpmock.Registry) {
+				r.Register(
+					httpmock.GraphQL(`query PullRequestByNumber\b`),
+					httpmock.StringResponse(`{"data":{"repository":{
+						"pullRequest":{"number":13}
+					}}}`))
+			},
+			wantPR:   13,
+			wantRepo: "https://example.org/OWNER/REPO",
+		},
 		{
 			name: "when provided branch argument with an open and closed PR for that branch name, it returns the open PR",
 			args: args{