feat: Add avatar stacks (#37594)

Parse `Co-authored-by:` trailers from commit messages and surface
contributors as an avatar stack across the commit page, commits list, PR
commits tab, latest-commit row, blame, graph, and dashboard feed.

- Up to 10 visible 20px avatars, GitHub-style overlap (6px first stride,
4px between subsequent), `+N` chip for the rest.
- Label: 1 → name; 2 → `<a> and <b>`; 3+ → `<N> people` opens a Tippy
popup with all participants.
- Names and avatars link to the repo's commits-by-author search; fall
back to profile or `mailto:`.
- Trailer parsing uses `net/mail.ParseAddress`, scans only the trailing
paragraph, filters out the commit's own author/committer.
- Drops the non-standard `Co-committed-by:` emission on squash merge and
web edits.

Devtest: `/devtest/coauthor-avatars`.

Fixes #25521

----
<img width="353" height="277" alt="image"
src="https://github.com/user-attachments/assets/72092ceb-97ca-4b09-9557-0b72d3c5458e"
/>

<img width="533" height="328"
src="https://github.com/user-attachments/assets/11d0c8f8-8b3f-4f2e-9993-879f1c06bcc5"
/>

---------

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>
Co-authored-by: silverwind <me@silverwind.io>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Giteabot <teabot@gitea.io>

modules/git/commit_message.go

@@ -0,0 +1,131 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package git
+
+import (
+	"net/mail"
+	"regexp"
+	"strings"
+	"sync"
+
+	"gitea.dev/modules/charset"
+	"gitea.dev/modules/container"
+	"gitea.dev/modules/util"
+)
+
+// CoAuthoredByTrailer is the canonical token for the `Co-authored-by:` git trailer.
+const CoAuthoredByTrailer = "Co-authored-by"
+
+type CommitIdentity struct {
+	Name  string
+	Email string
+}
+
+// CommitMessageTrailerValues keys are all in lower-case
+type CommitMessageTrailerValues map[string][]string
+
+type CommitMessage struct {
+	MessageRaw   string
+	messageUTF8  *string
+	messageTitle *string
+	messageBody  *string
+
+	trailerValues CommitMessageTrailerValues
+
+	allParticipants []*CommitIdentity
+}
+
+func (c *CommitMessage) MessageUTF8() string {
+	if c.messageUTF8 == nil {
+		bs := charset.ToUTF8(util.UnsafeStringToBytes(c.MessageRaw), charset.ConvertOpts{ErrorReplacement: []byte{'?'}})
+		c.messageUTF8 = new(util.UnsafeBytesToString(bs))
+	}
+	return *c.messageUTF8
+}
+
+func (c *CommitMessage) MessageTitle() string {
+	if c.messageTitle == nil {
+		s, _, _ := strings.Cut(strings.TrimSpace(c.MessageUTF8()), "\n")
+		c.messageTitle = new(strings.TrimSpace(s))
+	}
+	return *c.messageTitle
+}
+
+func (c *CommitMessage) MessageBody() string {
+	if c.messageBody == nil {
+		_, s, _ := strings.Cut(strings.TrimSpace(c.MessageUTF8()), "\n")
+		c.messageBody = new(strings.TrimSpace(s))
+	}
+	return *c.messageBody
+}
+
+func (c *CommitMessage) MessageTrailer() CommitMessageTrailerValues {
+	if c.trailerValues == nil {
+		_, _, trailer := CommitMessageSplitTrailer(c.MessageUTF8())
+		c.trailerValues = CommitMessageParseTrailer(trailer)
+	}
+	return c.trailerValues
+}
+
+var commitMessageTrailerSplit = sync.OnceValue(func() *regexp.Regexp {
+	// the sep is either something like "\n---\n" or "\n\n" in the body, or at the start of the body like "---\n"
+	return regexp.MustCompile(`(?s)^(?P<content>.*?)(?P<sep>^|^\n|^-{3,}\n|\n-{3,}\n|\n\n)(?P<trailer>(?:[A-Za-z0-9][-A-Za-z0-9]*:[^\n]*\n?)*)$`)
+})
+
+func CommitMessageSplitTrailer(s string) (content, sep, trailer string) {
+	s = util.NormalizeStringEOL(s)
+	re := commitMessageTrailerSplit()
+	v := re.FindStringSubmatch(s)
+	if v == nil {
+		return s, "", ""
+	}
+	return v[re.SubexpIndex("content")], v[re.SubexpIndex("sep")], v[re.SubexpIndex("trailer")]
+}
+
+func CommitMessageParseTrailer(s string) CommitMessageTrailerValues {
+	ret := CommitMessageTrailerValues{}
+	for line := range strings.SplitSeq(util.NormalizeStringEOL(s), "\n") {
+		k, v, ok := strings.Cut(line, ":")
+		if !ok {
+			continue
+		}
+		k, v = strings.TrimSpace(k), strings.TrimSpace(v)
+		kLower := strings.ToLower(k)
+		ret[kLower] = append(ret[kLower], v)
+	}
+	return ret
+}
+
+// AllParticipantIdentities returns all the participants in the commit, the first one is the commit's author
+func (c *Commit) AllParticipantIdentities() []*CommitIdentity {
+	if c.allParticipants != nil {
+		return c.allParticipants
+	}
+
+	exclude := container.Set[string]{}
+	c.allParticipants = append(c.allParticipants, &CommitIdentity{Name: c.Author.Name, Email: c.Author.Email})
+	exclude.Add(strings.ToLower(c.Author.Email))
+
+	addParticipant := func(name, email string) {
+		if name == "" && email == "" {
+			return
+		}
+		emailLower := strings.ToLower(email)
+		if emailLower != "" && exclude.Contains(emailLower) {
+			return
+		}
+		c.allParticipants = append(c.allParticipants, &CommitIdentity{Name: name, Email: email})
+		exclude.Add(emailLower)
+	}
+	addParticipant(c.Committer.Name, c.Committer.Email)
+	for _, coAuthorValue := range c.MessageTrailer()["co-authored-by"] {
+		addr, err := mail.ParseAddress(coAuthorValue)
+		if err == nil {
+			addParticipant(addr.Name, addr.Address)
+		} else {
+			addParticipant(coAuthorValue, "")
+		}
+	}
+	return c.allParticipants
+}