[PATCH 0 of 3] hgkeyring extension

[Markus Zapke-Gründemann]

This is the port of the mercurial_keyring extension to the core. The new extension is named hgkeyring.

It has the same functionality as the original extension. I removed some code for older versions of mercurial as well as the monkeypatch code. The extension uses now new hooks in the http modules. The hooks can also be used by similar extensions like factotum which still use monkeypatching.