Support for credential helpers

[0x2b3bfa0]

Follow-up to #768

it probably has a Git credentials helper that can provide the credentials - see https://www.git-scm.com/docs/gitcredentials

dulwich does not support reading credentials from gitcredentials helpers, but that is not properly hooked up yet in the porcelain unfortunately. If you try this again in a couple of months it may work. #768 (comment)

It would be really nice to support credential loading from helpers like in the standalone git command‑line tool. Many users rely on HTTPS remotes and their system keychain for passwordless authentication, especially on macOS.

Minimal example

>>> from dulwich.porcelain import ls_remote
>>> ls_remote("https://github.com/user/repository")

Expected output

{b'HEAD': b'···', b'refs/heads/main': b'···'}

Actual output

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/···/site-packages/dulwich/porcelain.py", line 1618, in ls_remote
    return client.get_refs(host_path)
  File "/···/site-packages/dulwich/client.py", line 2089, in get_refs
    refs, _, _ = self._discover_references(b"git-upload-pack", url)
  File "/···/site-packages/dulwich/client.py", line 1906, in _discover_references
    resp, read = self._http_request(url, headers, allow_compression=True)
  File "/···/site-packages/dulwich/client.py", line 1875, in _http_request
    raise HTTPUnauthorized(resp.getheader("WWW-Authenticate"), url)
dulwich.client.HTTPUnauthorized: No valid credentials provided

System configuration

$ uname -a
Darwin ··· 19.6.0 Darwin Kernel Version 19.6.0: Tue Nov 10 00:10:30 PST 2020; root:xnu-6153.141.10~1/RELEASE_X86_64 x86_64

$ python --version
Python 3.9.4

$ pip show dulwich
Name: dulwich
Version: 0.20.23
Summary: Python Git Library
Home-page: https://www.dulwich.io/
Author: Jelmer Vernooij
Author-email: [email protected]
License: Apachev2 or later or GPLv2
Location: /···/site-packages
Requires: certifi, urllib3
Required-by: dvc

$ cat /Applications/Xcode.app/Contents/Developer/usr/share/git-core/gitconfig
[credential]
        helper = osxkeychain

[jelmer]

This would be great to see implemented - it'd be great if somebody picked this up. I'm to help provide guidance and/or review pull requests.