Don't use go-git to parse branches

As with the previous commit, the code to parse `git config --get-regex` output
on our end isn't so bad.

pkg/commands/git_commands/branch_loader.go

@@ -9,7 +9,6 @@ import (
 	"time"
 
 	"github.com/jesseduffield/generics/set"
-	"github.com/jesseduffield/go-git/v5/config"
 	"github.com/jesseduffield/lazygit/pkg/commands/models"
 	"github.com/jesseduffield/lazygit/pkg/commands/oscommands"
 	"github.com/jesseduffield/lazygit/pkg/common"
@@ -30,7 +29,7 @@ import (
 // can just pull them out of here and put them there and then call them from in here
 
 type BranchLoaderConfigCommands interface {
-	Branches() (map[string]*config.Branch, error)
+	Branches(cmd oscommands.ICmdObjBuilder) map[string]*BranchConfig
 }
 
 type BranchInfo struct {
@@ -119,16 +118,13 @@ func (self *BranchLoader) Load(reflogCommits []*models.Commit,
 		branches = utils.Prepend(branches, &models.Branch{Name: info.RefName, DisplayName: info.DisplayName, Head: true, DetachedHead: info.DetachedHead, Recency: "  *"})
 	}
 
-	configBranches, err := self.config.Branches()
-	if err != nil {
-		return nil, err
-	}
+	configBranches := self.config.Branches(self.cmd)
 
 	for _, branch := range branches {
 		match := configBranches[branch.Name]
 		if match != nil {
 			branch.UpstreamRemote = match.Remote
-			branch.UpstreamBranch = match.Merge.Short()
+			branch.UpstreamBranch = match.Merge
 		}
 
 		// If the branch already existed, take over its BehindBaseBranch value

pkg/commands/git_commands/config.go

@@ -1,12 +1,20 @@
 package git_commands
 
 import (
+	"strings"
+
 	gogit "github.com/jesseduffield/go-git/v5"
-	"github.com/jesseduffield/go-git/v5/config"
 	"github.com/jesseduffield/lazygit/pkg/commands/git_config"
+	"github.com/jesseduffield/lazygit/pkg/commands/oscommands"
 	"github.com/jesseduffield/lazygit/pkg/common"
 )
 
+// BranchConfig holds the tracking configuration for a branch.
+type BranchConfig struct {
+	Remote string
+	Merge  string // short ref name of upstream branch
+}
+
 type ConfigCommands struct {
 	*common.Common
 
@@ -72,13 +80,39 @@ func (self *ConfigCommands) GetPushToCurrent() bool {
 }
 
 // returns the repo's branches as specified in the git config
-func (self *ConfigCommands) Branches() (map[string]*config.Branch, error) {
-	conf, err := self.repo.Config()
+func (self *ConfigCommands) Branches(cmd oscommands.ICmdObjBuilder) map[string]*BranchConfig {
+	cmdArgs := NewGitCmd("config").
+		Arg("--local", "--get-regexp", `^branch\.`).ToArgv()
+	output, err := cmd.New(cmdArgs).DontLog().RunWithOutput()
 	if err != nil {
-		return nil, err
+		// exit code 1 means no matching keys (no branches with config)
+		return nil
 	}
 
-	return conf.Branches, nil
+	result := make(map[string]*BranchConfig)
+	for _, line := range strings.Split(output, "\n") {
+		key, value, found := strings.Cut(strings.TrimSpace(line), " ")
+		if !found {
+			continue
+		}
+		// key is like "branch.<name>.remote" or "branch.<name>.merge"
+		lastDot := strings.LastIndex(key, ".")
+		if lastDot < 0 {
+			continue
+		}
+		configKey := key[lastDot+1:]
+		branchName := key[len("branch."):lastDot]
+		if _, ok := result[branchName]; !ok {
+			result[branchName] = &BranchConfig{}
+		}
+		switch configKey {
+		case "remote":
+			result[branchName].Remote = value
+		case "merge":
+			result[branchName].Merge = strings.TrimPrefix(value, "refs/heads/")
+		}
+	}
+	return result
 }
 
 func (self *ConfigCommands) GetGitFlowPrefixes() string {