fix(compare): show the diff when a comparison has no commits (#39247)

The compare page hid the diff when a comparison had no commits, even if
the trees differ. A direct comparison whose head is an ancestor of its
base hits this, which is where the Compare link on a force-push comment
leads after commits were dropped.

The diff box now shows whenever there are commits or changed files, with
the header reading "0 Commits".

Fixes: https://github.com/go-gitea/gitea/issues/36390
Co-authored-by: silverwind <me@silverwind.io>

tests/integration/compare_test.go

@@ -16,6 +16,7 @@ import (
 	"gitea.dev/modules/git"
 	"gitea.dev/modules/git/gitcmd"
 	"gitea.dev/modules/test"
+	"gitea.dev/modules/translation"
 	"gitea.dev/modules/util"
 	"gitea.dev/routers/common"
 	repo_service "gitea.dev/services/repository"
@@ -136,6 +137,15 @@ func TestCompareBranches(t *testing.T) {
 	diffChanges = []string{"test.txt"}
 
 	inspectCompare(t, htmlDoc, diffCount, diffChanges)
+
+	req = NewRequest(t, "GET", "/user2/repo20/compare/8babce967f21b9dfa6987f943b91093dac58a4f0..c8e31bc7688741a5287fcde4fbb8fc129ca07027")
+	resp = session.MakeRequest(t, req, http.StatusOK)
+	htmlDoc = NewHTMLParser(t, resp.Body)
+
+	inspectCompare(t, htmlDoc, 3, []string{"link_hi", "test.csv", "test.txt"})
+	assert.NotContains(t, htmlDoc.doc.Text(), translation.NewLocale("en-US").TrString("repo.commits.nothing_to_compare"))
+	commitsHeader := strings.Join(strings.Fields(htmlDoc.doc.Find(".ui.top.attached.header .flex-text-block").First().Text()), " ")
+	assert.Equal(t, "0 "+translation.NewLocale("en-US").TrString("repo.commits.commits"), commitsHeader)
 }
 
 func TestCompareWithRefSuffix(t *testing.T) {