phabricator: add a contrib script

The default Phabricator client arcanist is not friendly to send a stack of
changesets. It works better when a feature branch is reviewed as a single
review unit. However, we want multiple revisions per feature branch.

To be able to have an `hg email`-like UX to send and receive a stack of
commits easily, it seems we have to re-invent things. This patch adds
`phabricator.py` speaking Conduit API [1] in `contrib` as the first step.
This may also be an option for people who don't want to run PHP.

Config could be done in `hgrc` (instead of `arcrc` or `arcconfig`):

    [phabricator]
    # API token. Get it from https://phab.mercurial-scm.org/conduit/login/
    token = cli-xxxxxxxxxxxxxxxxxxxxxxxxxxxx
    url = https://phab.mercurial-scm.org/
    # callsign is used by the next patch
    callsign = HG

This patch only adds a single command: `debugcallconduit` to keep the patch
size small. To test it, having the above config, and run:

    $ hg debugcallconduit diffusion.repository.search <<EOF
    > {"constraints": {"callsigns": ["HG"]}}
    > EOF

The result will be printed in prettified JSON format.

[1]: Conduit APIs are listed at https://phab.mercurial-scm.org/conduit/

contrib/phabricator.py

@@ -0,0 +1,98 b''
+# phabricator.py - simple Phabricator integration
+#
+# Copyright 2017 Facebook, Inc.
+#
+# This software may be used and distributed according to the terms of the
+# GNU General Public License version 2 or any later version.
+"""simple Phabricator integration
+
+Config::
+
+    [phabricator]
+    # Phabricator URL
+    url = https://phab.example.com/
+
+    # API token. Get it from https://$HOST/conduit/login/
+    token = cli-xxxxxxxxxxxxxxxxxxxxxxxxxxxx
+"""
+
+from __future__ import absolute_import
+
+import json
+
+from mercurial.i18n import _
+from mercurial import (
+    error,
+    registrar,
+    url as urlmod,
+    util,
+)
+
+cmdtable = {}
+command = registrar.command(cmdtable)
+
+def urlencodenested(params):
+    """like urlencode, but works with nested parameters.
+
+    For example, if params is {'a': ['b', 'c'], 'd': {'e': 'f'}}, it will be
+    flattened to {'a[0]': 'b', 'a[1]': 'c', 'd[e]': 'f'} and then passed to
+    urlencode. Note: the encoding is consistent with PHP's http_build_query.
+    """
+    flatparams = util.sortdict()
+    def process(prefix, obj):
+        items = {list: enumerate, dict: lambda x: x.items()}.get(type(obj))
+        if items is None:
+            flatparams[prefix] = obj
+        else:
+            for k, v in items(obj):
+                if prefix:
+                    process('%s[%s]' % (prefix, k), v)
+                else:
+                    process(k, v)
+    process('', params)
+    return util.urlreq.urlencode(flatparams)
+
+def readurltoken(repo):
+    """return conduit url, token and make sure they exist
+
+    Currently read from [phabricator] config section. In the future, it might
+    make sense to read from .arcconfig and .arcrc as well.
+    """
+    values = []
+    section = 'phabricator'
+    for name in ['url', 'token']:
+        value = repo.ui.config(section, name)
+        if not value:
+            raise error.Abort(_('config %s.%s is required') % (section, name))
+        values.append(value)
+    return values
+
+def callconduit(repo, name, params):
+    """call Conduit API, params is a dict. return json.loads result, or None"""
+    host, token = readurltoken(repo)
+    url, authinfo = util.url('/'.join([host, 'api', name])).authinfo()
+    urlopener = urlmod.opener(repo.ui, authinfo)
+    repo.ui.debug('Conduit Call: %s %s\n' % (url, params))
+    params = params.copy()
+    params['api.token'] = token
+    request = util.urlreq.request(url, data=urlencodenested(params))
+    body = urlopener.open(request).read()
+    repo.ui.debug('Conduit Response: %s\n' % body)
+    parsed = json.loads(body)
+    if parsed.get(r'error_code'):
+        msg = (_('Conduit Error (%s): %s')
+               % (parsed[r'error_code'], parsed[r'error_info']))
+        raise error.Abort(msg)
+    return parsed[r'result']
+
+@command('debugcallconduit', [], _('METHOD'))
+def debugcallconduit(ui, repo, name):
+    """call Conduit API
+
+    Call parameters are read from stdin as a JSON blob. Result will be written
+    to stdout as a JSON blob.
+    """
+    params = json.loads(ui.fin.read())
+    result = callconduit(repo, name, params)
+    s = json.dumps(result, sort_keys=True, indent=2, separators=(',', ': '))
+    ui.write('%s\n' % s)