Use git rev-parse to check for existence of base branches

And cache the result locally in CommitLoader.

Author: stefanhaller

pkg/commands/git_commands/commit_loader.go

@@ -29,10 +29,11 @@ type CommitLoader struct {
 	*common.Common
 	cmd oscommands.ICmdObjBuilder
 
-	getRebaseMode func() (enums.RebaseMode, error)
-	readFile      func(filename string) ([]byte, error)
-	walkFiles     func(root string, fn filepath.WalkFunc) error
-	dotGitDir     string
+	getRebaseMode   func() (enums.RebaseMode, error)
+	readFile        func(filename string) ([]byte, error)
+	walkFiles       func(root string, fn filepath.WalkFunc) error
+	dotGitDir       string
+	doBranchesExist map[string]bool
 }
 
 // making our dependencies explicit for the sake of easier testing
@@ -43,12 +44,13 @@ func NewCommitLoader(
 	getRebaseMode func() (enums.RebaseMode, error),
 ) *CommitLoader {
 	return &CommitLoader{
-		Common:        cmn,
-		cmd:           cmd,
-		getRebaseMode: getRebaseMode,
-		readFile:      os.ReadFile,
-		walkFiles:     filepath.Walk,
-		dotGitDir:     dotGitDir,
+		Common:          cmn,
+		cmd:             cmd,
+		getRebaseMode:   getRebaseMode,
+		readFile:        os.ReadFile,
+		walkFiles:       filepath.Walk,
+		dotGitDir:       dotGitDir,
+		doBranchesExist: make(map[string]bool),
 	}
 }
 
@@ -381,11 +383,15 @@ func (self *CommitLoader) getMergeBase(refName string) string {
 }
 
 func (self *CommitLoader) branchExists(branchName string) bool {
-	// This is not the proper way to test whether a branch exists; it could fail
-	// in the future if git decides to store its branches in a different way.
-	// However, making git calls to test if the branches exist is too expensive.
-	_, err := os.Stat(filepath.Join(self.dotGitDir, "refs/heads", branchName))
-	return err == nil
+	branchExists, cacheEntryExists := self.doBranchesExist[branchName]
+	if cacheEntryExists {
+		return branchExists
+	}
+
+	err := self.cmd.New(fmt.Sprintf("git rev-parse --verify --quiet refs/heads/%s", self.cmd.Quote(branchName))).DontLog().Run()
+	branchExists = err == nil
+	self.doBranchesExist[branchName] = branchExists
+	return branchExists
 }
 
 func ignoringWarnings(commandOutput string) string {