fix(ui): too many participants shown in commit avatar stacks (#38689)

Show only author and co-authors without committer, and deduplicate the
same user with multiple email addresses.

The commit list "Author" column should not show the committer. This is
somewhat misleading, and arguably showing it on the individual commit
page is sufficient and consistent with other forges.

The same user with multiple email addresses often happens when
DEFAULT_KEEP_EMAIL_PRIVATE is enabled and Gitea does not use an actual
email address by default for edits. Showing the same avatar and name
twice is not helpful then.

Ref #37594
Fix #38488

---

Before
<img width="4088" height="2138" alt="before"
src="https://github.com/user-attachments/assets/a9e919f4-72d0-495f-a0b5-ec094f148f5a"
/>

After
<img width="4124" height="2142" alt="after"
src="https://github.com/user-attachments/assets/7212e527-ab83-440a-a8a4-95b5a79b3580"
/>

---------

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

services/repository/gitgraph/graph_models.go

@@ -109,7 +109,7 @@ func (graph *Graph) LoadAndProcessCommits(ctx context.Context, repository *repo_
 		if c.Commit.Author != nil {
 			emailSet.Add(c.Commit.Author.Email)
 		}
-		for _, sig := range c.Commit.AllParticipantIdentities() {
+		for _, sig := range c.Commit.AllAuthorIdentities() {
 			emailSet.Add(sig.Email)
 		}
 	}
@@ -125,7 +125,7 @@ func (graph *Graph) LoadAndProcessCommits(ctx context.Context, repository *repo_
 		}
 
 		c.User = emailUserMap.GetByEmail(c.Commit.Author.Email)
-		c.AvatarStackData = gituser.BuildAvatarStackData(ctx, c.Commit.AllParticipantIdentities(), emailUserMap)
+		c.AvatarStackData = gituser.BuildAvatarStackData(ctx, c.Commit.AllAuthorIdentities(), emailUserMap)
 
 		c.Verification = asymkey_service.ParseCommitWithSignature(ctx, c.Commit)