perf: reduce git processes and queries on pull request view (#40224)

Removes redundant work from the pull request conversation tab.

- Base and head refs resolve through the `cat-file` batch of the git
repos the callers already hold, instead of two `git rev-parse`
processes, reducing the number of git processes from 9 to 7 per page,
about 17 ms faster locally.
- Sidebar reviewers are batch-loaded and code-review comments reuse
their loaded poster, reducing the number of SQL queries from 387 to 358
on a PR with 169 timeline entries and 15 reviews.

Also fixes the commit list of merged pull requests, which resolved the
merge base as a branch name and returned 500.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/issues/review.go

@@ -121,11 +121,15 @@ func (rt ReviewType) Icon() string {
 
 // Review represents collection of code comments giving feedback for a PR
 type Review struct {
-	ID               int64 `xorm:"pk autoincr"`
-	Type             ReviewType
-	Reviewer         *user_model.User   `xorm:"-"`
-	ReviewerID       int64              `xorm:"index"`
-	ReviewerTeamID   int64              `xorm:"NOT NULL DEFAULT 0"`
+	ID       int64 `xorm:"pk autoincr"`
+	Type     ReviewType
+	Reviewer *user_model.User `xorm:"-"`
+
+	// HINT: REVIEWER-ID-NEGATIVE: ReviewerID and ReviewerTeamID must be positive at the moment,
+	// because the current template uses a trick "ItemID < 0" means "team id"
+	ReviewerID     int64 `xorm:"index"`
+	ReviewerTeamID int64 `xorm:"NOT NULL DEFAULT 0"`
+
 	ReviewerTeam     *organization.Team `xorm:"-"`
 	OriginalAuthor   string
 	OriginalAuthorID int64
@@ -186,7 +190,7 @@ func (r *Review) LoadReviewerTeam(ctx context.Context) (err error) {
 		return nil
 	}
 
-	r.ReviewerTeam, err = organization.GetTeamByID(ctx, r.ReviewerTeamID)
+	r.ReviewerTeamID, r.ReviewerTeam, err = organization.GetPossibleTeamByID(ctx, r.ReviewerTeamID)
 	return err
 }