gendoc: add support for loading extensions from config settings

We manage our installation and ship some extensions, enabled by default for our
users, that are in hgext3rd or other directories not scanned by this tool by
default. We want to generate docs during the build process, and having those
docs include the extensions that users don't have to manually enable is
desirable.

This is *not* desirable for the normal build process, however, and should never
be enabled by default.

Differential Revision: https://phab.mercurial-scm.org/D9989

doc/gendoc.py

@@ -330,6 +330,11 b' if __name__ == "__main__":'
         doc = encoding.strtolocal(sys.argv[1])
 
     ui = uimod.ui.load()
+    # Trigger extensions to load. This is disabled by default because it uses
+    # the current user's configuration, which is often not what is wanted.
+    if encoding.environ.get(b'GENDOC_LOAD_CONFIGURED_EXTENSIONS', b'0') != b'0':
+        extensions.loadall(ui)
+
     if doc == b'hg.1.gendoc':
         showdoc(ui)
     else: