feat: real user as commit author

Previously all commits were authored and committed by

    releaser-pleaser <>

This looked weird when looking at the commit. We now check with the
Forge API for details on the currently authenticated user, and use that
name and email as the commit author. The commit committer stays the same
for now.

In GitHub, the default `$GITHUB_TOKEN` does not allow access to the
required endpoint, so for github the user `github-actions[bot]
<41898282+github-actions[bot]@users.noreply.github.com>` is hardcoded
when the request fails.

internal/git/git_test.go

@@ -0,0 +1,46 @@
+package git
+
+import (
+	"reflect"
+	"strconv"
+	"testing"
+	"time"
+
+	"github.com/go-git/go-git/v5/plumbing/object"
+)
+
+func TestAuthor_signature(t *testing.T) {
+	now := time.Now()
+
+	tests := []struct {
+		author Author
+		want   *object.Signature
+	}{
+		{author: Author{Name: "foo", Email: "bar@example.com"}, want: &object.Signature{Name: "foo", Email: "bar@example.com", When: now}},
+		{author: Author{Name: "bar", Email: "foo@example.com"}, want: &object.Signature{Name: "bar", Email: "foo@example.com", When: now}},
+	}
+	for i, tt := range tests {
+		t.Run(strconv.FormatInt(int64(i), 10), func(t *testing.T) {
+			if got := tt.author.signature(now); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("signature() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}
+
+func TestAuthor_String(t *testing.T) {
+	tests := []struct {
+		author Author
+		want   string
+	}{
+		{author: Author{Name: "foo", Email: "bar@example.com"}, want: "foo <bar@example.com>"},
+		{author: Author{Name: "bar", Email: "foo@example.com"}, want: "bar <foo@example.com>"},
+	}
+	for i, tt := range tests {
+		t.Run(strconv.FormatInt(int64(i), 10), func(t *testing.T) {
+			if got := tt.author.String(); !reflect.DeepEqual(got, tt.want) {
+				t.Errorf("String() = %v, want %v", got, tt.want)
+			}
+		})
+	}
+}