fix: consider HEAD requests to be pulls (#6750)

Previously an anonymous GET request to e.g.
https://codeberg.org/forgejo/forgejo/HEAD was allowed, as GET requests
are considered pulls and those don't need authentication for a public
repository, but a HEAD request to the same URL was rejected with a 401.
Since the result of a HEAD request is a subset of the result of a GET
request it is safe to allow HEAD as well.

This isn't really a practical issue for Forgejo itself, but I have encountered this in https://codeberg.org/forgejo-aneksajo/forgejo-aneksajo/issues/40. Since the fix isn't git-annex specific I am proposing it here.

## Checklist

The [contributor guide](https://forgejo.org/docs/next/contributor/) contains information that will be helpful to first time contributors. There also are a few [conditions for merging Pull Requests in Forgejo repositories](https://codeberg.org/forgejo/governance/src/branch/main/PullRequestsAgreement.md). You are also welcome to join the [Forgejo development chatroom](https://matrix.to/#/#forgejo-development:matrix.org).

### Tests

- I added test coverage for Go changes...
  - [ ] in their respective `*_test.go` for unit tests.
  - [x] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes...
  - [ ] in `web_src/js/*.test.js` if it can be unit tested.
  - [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).

### Documentation

- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [X] I did not document these changes and I do not expect someone else to do it.

### Release notes

- [x] I do not want this change to show in the release notes.
- [ ] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6750
Reviewed-by: Michael Kriese <michael.kriese@gmx.de>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Matthias Riße <m.risse@fz-juelich.de>
Co-committed-by: Matthias Riße <m.risse@fz-juelich.de>

routers/web/repo/githttp.go

@@ -78,7 +78,7 @@ func httpBase(ctx *context.Context) *serviceHandler {
 		strings.HasSuffix(ctx.Req.URL.Path, "git-upload-archive") {
 		isPull = true
 	} else {
-		isPull = ctx.Req.Method == "GET"
+		isPull = ctx.Req.Method == "GET" || ctx.Req.Method == "HEAD"
 	}
 
 	var accessMode perm.AccessMode