tests: verify code in _examples

A new test has been added in order to verify that all code under
_examples is up to date and has not broken due to API changes.

This test runs as the last step of the test.yml CI workflow.

Closes go-git#912.

.github/workflows/test.yml

@@ -34,3 +34,8 @@ jobs:
 
     - name: Test
       run: make test-coverage
+
+    - name: Verify Examples
+      env:
+        EXAMPLES_DIR: _examples
+      run: go test -test.v -test.run=^TestBuildExamples$

build_examples_test.go

@@ -0,0 +1,38 @@
+package git
+
+import (
+	"os"
+	"os/exec"
+	"path/filepath"
+	"testing"
+)
+
+func TestBuildExamples(t *testing.T) {
+	examplesDir := os.Getenv("EXAMPLES_DIR")
+	if len(examplesDir) == 0 {
+		t.Skip("EXAMPLES_DIR not set")
+	}
+
+	examplesDir, err := filepath.Abs(examplesDir)
+	if err != nil {
+		t.Fatal(err.Error())
+	}
+
+	glob := filepath.Join(examplesDir, "/**/main.go")
+	matches, err := filepath.Glob(glob)
+	for _, match := range matches {
+		testName, err := filepath.Rel(examplesDir, match)
+		if err != nil {
+			t.Fatal(err.Error())
+		}
+
+		testName = filepath.Dir(testName)
+		t.Run(testName, func(t *testing.T) {
+			cmd := exec.Command("go", "build", "-o", os.DevNull)
+			cmd.Dir = filepath.Dir(match)
+			if err := cmd.Run(); err != nil {
+				t.Error(err.Error())
+			}
+		})
+	}
+}