Refactor error handling in ReadBranchConfig to avoid panic

git/client.go

@@ -391,11 +391,13 @@ func (c *Client) ReadBranchConfig(ctx context.Context, branch string) (BranchCon
 
 	out, err := cmd.Output()
 	if err != nil {
-		// This will error if no matches are found but the git command still ran successfully. We only
-		// want to return an error if the command failed to run, usually and ExitError, which will be
-		// indicated by output on Stderr.
-		if err.(*GitError).Stderr != "" {
-			return BranchConfig{}, err
+		// This is the error we expect if the git command does not run successfully.
+		// Note: err is non-nil if the command is successful but has no output
+		var gitError *GitError
+		if errors.As(err, &gitError) {
+			if gitError.Stderr != "" {
+				return BranchConfig{}, err
+			}
 		}
 		return BranchConfig{}, nil
 	}

git/client_test.go

@@ -750,7 +750,7 @@ func TestClientReadBranchConfig(t *testing.T) {
 			cmdStderr:        "git error message",
 			branch:           "trunk",
 			wantBranchConfig: BranchConfig{},
-			wantError:        &GitError{ExitCode: 1, Stderr: "git error message"},
+			wantError:        &GitError{},
 		},
 		{
 			name:             "git config runs successfully but returns no output",
@@ -773,12 +773,10 @@ func TestClientReadBranchConfig(t *testing.T) {
 			wantCmdArgs := fmt.Sprintf("path/to/git config --get-regexp ^branch\\.%s\\.(remote|merge|gh-merge-base)$", tt.branch)
 			assert.Equal(t, wantCmdArgs, strings.Join(cmd.Args[3:], " "))
 			assert.Equal(t, tt.wantBranchConfig, branchConfig)
-			if err != nil {
-				if tt.wantError == nil {
-					t.Fatalf("expected no error but got %v", err)
-				}
-				assert.Equal(t, tt.wantError.ExitCode, err.(*GitError).ExitCode)
-				assert.Equal(t, tt.wantError.Stderr, err.(*GitError).Stderr)
+			if tt.wantError != nil {
+				assert.ErrorAs(t, err, &tt.wantError)
+			} else {
+				assert.NoError(t, err)
 			}
 		})
 	}