help: add internals.extensions topic
I think it's better to include the API overview in core as we now have the
internals section in our help system.
Retrieved from the wiki, and formatted as reST. Several wiki links are
removed since they are invalid in the help. The sections about example
extension are removed at all as they seemed too verbose.
https://www.mercurial-scm.org/wiki/WritingExtensions
#require serve
$ hg clone http://localhost:$HGPORT/ copy
abort: * (glob)
[255]
$ test -d copy
[1]
$ "$PYTHON" "$TESTDIR/dumbhttp.py" -p $HGPORT --pid dumb.pid
$ cat dumb.pid >> $DAEMON_PIDS
$ hg clone http://localhost:$HGPORT/foo copy2
abort: HTTP Error 404: * (glob)
[255]
$ killdaemons.py