Fix missing commit message body when the message has leading newlines (#25418)

Commit with `echo "\nmessage after a blank line\nsecond line of the
message" | git commit --cleanup=verbatim -F -` and push.

<img width="1139" alt="image"
src="https://github.com/go-gitea/gitea/assets/9418365/f9a2c28c-e307-4c78-9e31-3d3ace7b9274">

modules/templates/util_render_test.go

@@ -0,0 +1,56 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package templates
+
+import (
+	"context"
+	"html/template"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestRenderCommitBody(t *testing.T) {
+	type args struct {
+		ctx       context.Context
+		msg       string
+		urlPrefix string
+		metas     map[string]string
+	}
+	tests := []struct {
+		name string
+		args args
+		want template.HTML
+	}{
+		{
+			name: "multiple lines",
+			args: args{
+				ctx: context.Background(),
+				msg: "first line\nsecond line",
+			},
+			want: "second line",
+		},
+		{
+			name: "multiple lines with leading newlines",
+			args: args{
+				ctx: context.Background(),
+				msg: "\n\n\n\nfirst line\nsecond line",
+			},
+			want: "second line",
+		},
+		{
+			name: "multiple lines with trailing newlines",
+			args: args{
+				ctx: context.Background(),
+				msg: "first line\nsecond line\n\n\n",
+			},
+			want: "second line",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equalf(t, tt.want, RenderCommitBody(tt.args.ctx, tt.args.msg, tt.args.urlPrefix, tt.args.metas), "RenderCommitBody(%v, %v, %v, %v)", tt.args.ctx, tt.args.msg, tt.args.urlPrefix, tt.args.metas)
+		})
+	}
+}