Use batch database operations instead of one by one to optimze api pulls (#32680)

Resolve #31492

The response time for the Pull Requests API has improved significantly,
dropping from over `2000ms` to about `350ms` on my local machine. It's
about `6` times faster.

A key area for further optimization lies in batch-fetching data for
`apiPullRequest.ChangedFiles, apiPullRequest.Additions, and
apiPullRequest.Deletions`.

Tests `TestAPIViewPulls` does exist and new tests added.

- This PR also fixes some bugs in `GetDiff` functions.
- This PR also fixes data inconsistent in test data. For a pull request,
the head branch's reference should be equal to the reference in
`pull/xxx/head`.

tests/integration/api_pull_commits_test.go

@@ -33,8 +33,8 @@ func TestAPIPullCommits(t *testing.T) {
 		return
 	}
 
-	assert.Equal(t, "5f22f7d0d95d614d25a5b68592adb345a4b5c7fd", commits[0].SHA)
-	assert.Equal(t, "4a357436d925b5c974181ff12a994538ddc5a269", commits[1].SHA)
+	assert.Equal(t, "985f0301dba5e7b34be866819cd15ad3d8f508ee", commits[0].SHA)
+	assert.Equal(t, "5c050d3b6d2db231ab1f64e324f1b6b9a0b181c2", commits[1].SHA)
 
 	assert.NotEmpty(t, commits[0].Files)
 	assert.NotEmpty(t, commits[1].Files)