Add DEFAULT_TITLE_SOURCE setting for pull request title default behavior

[0xGREG]

Adds a new DEFAULT_TITLE_SOURCE option under [repository.pull-request] with three values:

• first-commit (default): uses the oldest commit summary, current behavior since v1.26
• auto: normalizes branch name as title for multi-commit PRs (just like GitHub), use commit summary for single-commit PRs

Closes #37463

[silverwind]

I tested the auto transformation against GitHub's web UI by creating ~20 branches in a test repo and reading the title placeholder GitHub renders on the compare page. The current implementation deviates from real GitHub behavior in several cases:

Branch	GitHub	This PR
fix/the-bug	Fix/the bug	Fix/the bug ✓
Already-Capitalized	Already capitalized	Already Capitalized
ALL-CAPS-BRANCH	All caps branch	ALL CAPS BRANCH
MixedCase-Name	Mixed case name	MixedCase Name
fooBar-baz	Foo bar baz	fooBar baz
foo/BAR	Foo/bar	foo/BAR
_leading-underscore	Leading underscore	<space>leading underscore
CamelCase	Camel case	CamelCase
foo--double-dash	Foo<sp><sp>double dash	Foo<sp><sp>double dash ✓
123-fix	123 fix	123 fix ✓

GitHub's full algorithm (derived empirically)

1. Split camelCase/PascalCase: insert a space before any uppercase letter that follows a lowercase letter (fooBar → foo Bar, MixedCase → Mixed Case).
2. Replace - and _ with single spaces — every occurrence, consecutive separators are not collapsed (foo--bar → foo bar).
3. Lowercase the entire string.
4. Left-trim whitespace (so a leading _/- is dropped, not turned into a leading space). Trailing separators are not trimmed (trailing- → Trailing<space>).
5. Capitalize the first letter that remains.
6. Slashes, dots, digits, emoji, CJK chars are preserved as-is.

A reference Go implementation:

func humanizeBranchName(s string) string {
    var b strings.Builder
    runes := []rune(s)
    for i, r := range runes {
        if i > 0 && unicode.IsUpper(r) && unicode.IsLower(runes[i-1]) {
            b.WriteByte(' ')
        }
        if r == '-' || r == '_' {
            b.WriteByte(' ')
        } else {
            b.WriteRune(unicode.ToLower(r))
        }
    }
    out := strings.TrimLeft(b.String(), " ")
    if out == "" {
        return out
    }
    outRunes := []rune(out)
    outRunes[0] = unicode.ToUpper(outRunes[0])
    return string(outRunes)
}

---

Written with the help of Claude Opus 4.7

[lautriva]

4. Left-trim whitespace (so a leading _/- is dropped, not turned into a leading space). Trailing separators are not trimmed (trailing- → Trailing<space>)

Do we still want to keep this behavior of left-trim but not right-trim?

Look like a Github bug for me

[silverwind]

Look like a Github bug for me

Yeah, unequal trimming seems like bug, I'll push a fixup for this.

[TheFox0x7]

Shouldn't this be a per repository/org setting, rather or in addition to the global one?

[wxiaoguang]

Shouldn't this be a per repository/org setting, rather or in addition to the global one?

Well, a longstanding problem. It needs a flexible configuration system.

However, either some people can't make a right design or implementation, or some people don't have time or interests.

[silverwind]

Any such settings would benefit from a config chain like global -> org/user -> repo, but it needs a proper designed inheritance system. Having this global setting now won't hurt a future design.

@0xGREG could you file a pr against https://gitea.com/gitea/docs for the docs update please?

[wxiaoguang]

Fixed more cases:

• {"FixHTMLBug", "Fix html bug"}
• {"foo--double-dash", "Foo double dash"},

[0xGREG]

Created PR for the docs https://gitea.com/gitea/docs/pulls/391

[GiteaBot]

@0xGREG please fix the merge conflicts. 🍵

[bircni]

@0xGREG please fix the merge conflicts. 🍵

done

[wxiaoguang]

Oh, I have removed them .......

They are useless template variables.

Maybe need more clean up for the "compare diff" page (it is one of the most messy pages, just like "pull request view" page)

[wxiaoguang]

-> Refactor compare diff/pull page (1) #37481