enhance!: raise minimum git version to 2.34 (#39565)

Raise the minimum git version to 2.34, the version in Ubuntu 22.04,
Debian 12 and RHEL 8 ship newer, and remove the fallbacks it makes
obsolete.

- Always enable AGit
- Use `diff --skip-to` and `apply -3` unconditionally
- Set the default branch of new repos and wikis via `git init
--initial-branch`
- Detect rebase conflicts via `REBASE_HEAD`

tests/integration/wiki_test.go

@@ -40,11 +40,6 @@ func TestRepoWikiPages(t *testing.T) {
 }
 
 func testRepoWikiCloneHTTP(t *testing.T, u *url.URL) {
-	// When proc-receive support is enabled globally, the HTTP receive-pack pre-check
-	// must still require write access for wiki repositories. Exercise this with a
-	// normal wiki push because the regression is about the pre-check, not agit refs.
-	require.True(t, git.DefaultFeatures().SupportProcReceive) // modern git should all support proc-receive
-
 	wikiURL := *u
 	wikiURL.Path = "/user2/repo1.wiki.git"