Prevent NPE on partial match of compare URL and allow short SHA1 compare URLs (#18472)

* Don't panic & allow shorter sha1

- Don't panic when the full regex isn't matched and allow the usage of a
shorter sha1 being used.
- Resolves #18471

* Update modules/markup/html.go

Co-authored-by: zeripath <art27@cantab.net>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>

modules/markup/html_test.go

@@ -548,3 +548,16 @@ func TestFuzz(t *testing.T) {
 
 	assert.NoError(t, err)
 }
+
+func TestIssue18471(t *testing.T) {
+	data := `http://domain/org/repo/compare/783b039...da951ce`
+
+	var res strings.Builder
+	err := PostProcess(&RenderContext{
+		URLPrefix: "https://example.com",
+		Metas:     localMetas,
+	}, strings.NewReader(data), &res)
+
+	assert.NoError(t, err)
+	assert.Equal(t, res.String(), "<a href=\"http://domain/org/repo/compare/783b039...da951ce\" class=\"compare\"><code class=\"nohighlight\">783b039...da951ce</code></a>")
+}