Use regular 'git branch' call instead of using --show-current

Older versions of git don't support --show-current flag.

commit-id:e54c8b99

github/githubclient/client.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"regexp"
+	"strings"
 
 	"github.com/ejoffe/spr/config"
 	"github.com/ejoffe/spr/git"
@@ -87,9 +88,7 @@ func (c *client) GetInfo(ctx context.Context, gitcmd git.GitInterface) *github.G
 	err := c.api.Query(ctx, &query, variables)
 	check(err)
 
-	var branchname string
-	err = gitcmd.Git("branch --show-current", &branchname)
-	check(err)
+	branchname := getLocalBranchName(gitcmd)
 
 	var requests []*github.PullRequest
 	for _, node := range query.Viewer.PullRequests.Nodes {
@@ -325,6 +324,19 @@ func (c *client) ClosePullRequest(ctx context.Context, pr *github.PullRequest) {
 	}
 }
 
+func getLocalBranchName(gitcmd git.GitInterface) string {
+	var output string
+	err := gitcmd.Git("branch", &output)
+	check(err)
+	lines := strings.Split(output, "\n")
+	for _, line := range lines {
+		if strings.HasPrefix(line, "* ") {
+			return line[2:]
+		}
+	}
+	panic("cannot determine local git branch name")
+}
+
 func branchNameFromCommit(info *github.GitHubInfo, commit git.Commit) string {
 	return "pr/" + info.UserName + "/" + info.LocalBranch + "/" + commit.CommitID
 }