pull: add an option to use rebase for merging pull requests

For DVCS, either merge or rebase works for getting new code in a pull
request in the main branch.

The rebase workflow produces a linear history which is cleaner, and
more bisect-able.

This commit adds a repo-level option to enable the rebase workflow. Once
enabled, "Merge Pull Request" will be replaced by
"Rebase and Merge Pull Request" which does exactly what the user wants.
It's unlikely a project wants a mixed-use of both rebase and merge
workflows, therefore the feature is not implemented as a drop-down
button like what GitHub does
(https://github.com/blog/2243-rebase-and-merge-pull-requests).

2 files changed, 23 insertions, 13 deletions

diff --git a/models/pull.go b/models/pull.go
index 27a843d5..bc03c804 100644
--- a/models/pull.go
+++ b/models/pull.go
@@ -233,31 +233,40 @@ func (pr *PullRequest) Merge(doer *User, baseGitRepo *git.Repository) (err error
 		return fmt.Errorf("git remote add [%s -> %s]: %s", headRepoPath, tmpBasePath, stderr)
 	}
 
-	// Merge commits.
 	if _, stderr, err = process.ExecDir(-1, tmpBasePath,
 		fmt.Sprintf("PullRequest.Merge (git fetch): %s", tmpBasePath),
 		"git", "fetch", "head_repo"); err != nil {
 		return fmt.Errorf("git fetch [%s -> %s]: %s", headRepoPath, tmpBasePath, stderr)
 	}
 
-	if _, stderr, err = process.ExecDir(-1, tmpBasePath,
-		fmt.Sprintf("PullRequest.Merge (git merge --no-ff --no-commit): %s", tmpBasePath),
-		"git", "merge", "--no-ff", "--no-commit", "head_repo/"+pr.HeadBranch); err != nil {
-		return fmt.Errorf("git merge --no-ff --no-commit [%s]: %v - %s", tmpBasePath, err, stderr)
-	}
+	if (pr.BaseRepo.PullUseRebase) {
+		// Rebase.
+		if _, stderr, err = process.ExecDir(-1, tmpBasePath,
+			fmt.Sprintf("PullRequest.Rebase (git rebase): %s", tmpBasePath),
+			"git", "rebase", "-q", pr.BaseBranch, "head_repo/"+pr.HeadBranch); err != nil {
+			return fmt.Errorf("git rebase [%s -> %s]: %s", headRepoPath, tmpBasePath, stderr)
+		}
+	} else {
+		// Merge commits.
+		if _, stderr, err = process.ExecDir(-1, tmpBasePath,
+			fmt.Sprintf("PullRequest.Merge (git merge --no-ff --no-commit): %s", tmpBasePath),
+			"git", "merge", "--no-ff", "--no-commit", "head_repo/"+pr.HeadBranch); err != nil {
+			return fmt.Errorf("git merge --no-ff --no-commit [%s]: %v - %s", tmpBasePath, err, stderr)
+		}
 
-	sig := doer.NewGitSig()
-	if _, stderr, err = process.ExecDir(-1, tmpBasePath,
-		fmt.Sprintf("PullRequest.Merge (git merge): %s", tmpBasePath),
-		"git", "commit", fmt.Sprintf("--author='%s <%s>'", sig.Name, sig.Email),
-		"-m", fmt.Sprintf("Merge branch '%s' of %s/%s into %s", pr.HeadBranch, pr.HeadUserName, pr.HeadRepo.Name, pr.BaseBranch)); err != nil {
-		return fmt.Errorf("git commit [%s]: %v - %s", tmpBasePath, err, stderr)
+		sig := doer.NewGitSig()
+		if _, stderr, err = process.ExecDir(-1, tmpBasePath,
+			fmt.Sprintf("PullRequest.Merge (git merge): %s", tmpBasePath),
+			"git", "commit", fmt.Sprintf("--author='%s <%s>'", sig.Name, sig.Email),
+			"-m", fmt.Sprintf("Merge branch '%s' of %s/%s into %s", pr.HeadBranch, pr.HeadUserName, pr.HeadRepo.Name, pr.BaseBranch)); err != nil {
+			return fmt.Errorf("git commit [%s]: %v - %s", tmpBasePath, err, stderr)
+		}
 	}
 
 	// Push back to upstream.
 	if _, stderr, err = process.ExecDir(-1, tmpBasePath,
 		fmt.Sprintf("PullRequest.Merge (git push): %s", tmpBasePath),
-		"git", "push", baseGitRepo.Path, pr.BaseBranch); err != nil {
+		"git", "push", "head_repo", "HEAD:"+pr.BaseBranch); err != nil {
 		return fmt.Errorf("git push: %s", stderr)
 	}
 
diff --git a/models/repo.go b/models/repo.go
index 998914d5..7e9bd916 100644
--- a/models/repo.go
+++ b/models/repo.go
@@ -184,6 +184,7 @@ type Repository struct {
 	ExternalTrackerStyle  string
 	ExternalMetas         map[string]string `xorm:"-"`
 	EnablePulls           bool              `xorm:"NOT NULL DEFAULT true"`
+	PullUseRebase         bool              `xorm:"NOT NULL DEFAULT false"`
 
 	IsFork   bool `xorm:"NOT NULL DEFAULT false"`
 	ForkID   int64