fix: correctly get stats for API commits (#8756)

- Instead of generating a patch and parsing its contents, use a faster and simple way to get it via `--shortstat`.
- Resolves forgejo/forgejo#8725
- Regression of forgejo/forgejo#7682
- Adds unit test.
- Adds integration test.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8756
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

tests/integration/api_repo_git_commits_test.go

@@ -231,3 +231,22 @@ func TestGetFileHistoryNotOnMaster(t *testing.T) {
 
 	assert.Equal(t, "1", resp.Header().Get("X-Total"))
 }
+
+func TestAPIReposGitCommit(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	session := loginUser(t, "user2")
+	token := getTokenForLoggedInUser(t, session, auth_model.AccessTokenScopeReadRepository)
+
+	req := NewRequest(t, "GET", "/api/v1/repos/user2/repo16/git/commits/f27c2b2b03dcab38beaf89b0ab4ff61f6de63441").
+		AddTokenAuth(token)
+	resp := MakeRequest(t, req, http.StatusOK)
+
+	var apiData api.Commit
+	DecodeJSON(t, resp, &apiData)
+
+	assert.Equal(t, "f27c2b2b03dcab38beaf89b0ab4ff61f6de63441", apiData.CommitMeta.SHA)
+	assert.Equal(t, 1, apiData.Stats.Total)
+	assert.Equal(t, 1, apiData.Stats.Additions)
+	assert.Equal(t, 0, apiData.Stats.Deletions)
+}