config: add configuration for PR merge method

The GitHub pull request API support squash, rebase, and merge. As
explored in issue #195, with signed commits required on the target
branch, it's not possible to use the rebase due to a known GitHub
implementation issue.

This change adds a new per-repo string configuration attribute,
`mergeMethod`, `"rebase"` by default. The current behaviour is captured by
this default value.

.spr.yml

@@ -5,3 +5,4 @@ requireChecks: true
 requireApproval: true
 githubRemote: origin
 githubBranch: master
+mergeMethod: rebase

config/config.go

@@ -10,6 +10,7 @@ import (
 
 	"github.com/ejoffe/rake"
 	"github.com/ejoffe/spr/git"
+	"github.com/shurcooL/githubv4"
 )
 
 type Config struct {
@@ -28,6 +29,7 @@ type RepoConfig struct {
 
 	GitHubRemote string `default:"origin" yaml:"githubRemote"`
 	GitHubBranch string `default:"master" yaml:"githubBranch"`
+	MergeMethod  string `default:"rebase" yaml:"mergeMethod"`
 }
 
 type UserConfig struct {
@@ -121,6 +123,25 @@ func GitHubRemoteSource(config *Config, gitcmd git.GitInterface) *remoteSource {
 	}
 }
 
+func (c Config) MergeMethod() (githubv4.PullRequestMergeMethod, error) {
+	var mergeMethod githubv4.PullRequestMergeMethod
+	var err error
+	switch strings.ToLower(c.Repo.MergeMethod) {
+	case "merge":
+		mergeMethod = githubv4.PullRequestMergeMethodMerge
+	case "squash":
+		mergeMethod = githubv4.PullRequestMergeMethodSquash
+	case "rebase", "":
+		mergeMethod = githubv4.PullRequestMergeMethodRebase
+	default:
+		err = fmt.Errorf(
+			`unknown merge method %q, choose from "merge", "squash", or "rebase"`,
+			c.Repo.MergeMethod,
+		)
+	}
+	return mergeMethod, err
+}
+
 type remoteSource struct {
 	gitcmd git.GitInterface
 	config *Config

config/config_test.go

@@ -4,6 +4,7 @@ import (
 	"testing"
 
 	"github.com/ejoffe/spr/git/mockgit"
+	"github.com/shurcooL/githubv4"
 	"github.com/stretchr/testify/assert"
 )
 
@@ -72,6 +73,7 @@ func TestDefaultConfig(t *testing.T) {
 			RequireApproval: true,
 			GitHubRemote:    "origin",
 			GitHubBranch:    "master",
+			MergeMethod:     "rebase",
 		},
 		User: &UserConfig{
 			ShowPRLink:       true,
@@ -100,6 +102,7 @@ func TestGitHubRemoteSource(t *testing.T) {
 			RequireApproval: false,
 			GitHubRemote:    "",
 			GitHubBranch:    "",
+			MergeMethod:     "",
 		},
 		User: &UserConfig{
 			ShowPRLink:       false,
@@ -119,3 +122,44 @@ func TestGitHubRemoteSource(t *testing.T) {
 	source.Load(nil)
 	assert.Equal(t, expect, actual)
 }
+
+func TestMergeMethodHelper(t *testing.T) {
+	for _, tc := range []struct {
+		configValue string
+		expected    githubv4.PullRequestMergeMethod
+	}{
+		{
+			configValue: "rebase",
+			expected:    githubv4.PullRequestMergeMethodRebase,
+		},
+		{
+			configValue: "",
+			expected:    githubv4.PullRequestMergeMethodRebase,
+		},
+		{
+			configValue: "Merge",
+			expected:    githubv4.PullRequestMergeMethodMerge,
+		},
+		{
+			configValue: "SQUASH",
+			expected:    githubv4.PullRequestMergeMethodSquash,
+		},
+	} {
+		tcName := tc.configValue
+		if tcName == "" {
+			tcName = "<EMPTY>"
+		}
+		t.Run(tcName, func(t *testing.T) {
+			config := &Config{Repo: &RepoConfig{MergeMethod: tc.configValue}}
+			actual, err := config.MergeMethod()
+			assert.NoError(t, err)
+			assert.Equal(t, tc.expected, actual)
+		})
+	}
+	t.Run("invalid", func(t *testing.T) {
+		config := &Config{Repo: &RepoConfig{MergeMethod: "magic"}}
+		actual, err := config.MergeMethod()
+		assert.Error(t, err)
+		assert.Empty(t, actual)
+	})
+}

readme.md

@@ -137,6 +137,8 @@ MERGED #60 Feature 3
 
 To merge only part of the stack use the **--upto** flag with the top pull request number in the stack that you would like to merge.
 
+By default merges are done using the rebase merge method, this can be changed using the mergeMethod configuration.
+
 ```shell
 > git spr merge --upto 59
 MERGED #58 Feature 1
@@ -160,6 +162,7 @@ User specific configuration is saved to .spr.yml in the user home directory.
 | githubRemote        | str  | origin     | github remote name to use                                      |
 | githubBranch        | str  | master     | github branch for pull request target                          |
 | githubHost          | str  | github.com | github host, can be updated for github enterprise usecase      |
+| mergeMethod         | str  | rebase     | merge method, valid values: [rebase, squash, merge]            |
 
 | User Config         | Type | Default | Description                                                       |
 | ------------------- | ---- | ------- | ----------------------------------------------------------------- |