errors: add a function to extract exit status from error

Right now, we have some custom code to extract the exit status from an
appropriate error.  However, we'll need to use this in multiple places,
so let's refactor it out into a function.

We use the Go 1.13 errors package because we no longer test for Go 1.13
since our dependencies also use this package and there's no practical
way to avoid this dependency.

errors/types.go

@@ -1,9 +1,12 @@
 package errors
 
 import (
+	goerrors "errors"
 	"fmt"
 	"net/url"
+	"os/exec"
 	"strconv"
+	"syscall"
 	"time"
 
 	"github.com/pkg/errors"
@@ -487,3 +490,14 @@ func parentOf(err error) error {
 
 	return nil
 }
+
+func ExitStatus(err error) int {
+	var eerr *exec.ExitError
+	if goerrors.As(err, &eerr) {
+		ws, ok := eerr.ProcessState.Sys().(syscall.WaitStatus)
+		if ok {
+			return ws.ExitStatus()
+		}
+	}
+	return -1
+}

git/git.go

@@ -14,13 +14,11 @@ import (
 	"io/ioutil"
 	"net/url"
 	"os"
-	"os/exec"
 	"path/filepath"
 	"regexp"
 	"strconv"
 	"strings"
 	"sync"
-	"syscall"
 	"time"
 
 	lfserrors "github.com/git-lfs/git-lfs/v3/errors"
@@ -733,13 +731,9 @@ func GitAndRootDirs() (string, string, error) {
 		// If we got a fatal error, it's possible we're on a newer
 		// (2.24+) Git and we're not in a worktree, so fall back to just
 		// looking up the repo directory.
-		if e, ok := err.(*exec.ExitError); ok {
-			var ws syscall.WaitStatus
-			ws, ok = e.ProcessState.Sys().(syscall.WaitStatus)
-			if ok && ws.ExitStatus() == 128 {
-				absGitDir, err := GitDir()
-				return absGitDir, "", err
-			}
+		if lfserrors.ExitStatus(err) == 128 {
+			absGitDir, err := GitDir()
+			return absGitDir, "", err
 		}
 		return "", "", fmt.Errorf("failed to call git rev-parse --git-dir --show-toplevel: %q", buf.String())
 	}