phabricator: check associated Differential Revision from commit message

Previously, only tags can "associate" a changeset to a Differential
Revision. But the usual pattern (arc patch or hg phabread) is to put the
Differential Revision URL in commit message.

This patch makes the code read commit message to find associated
Differential Revision if associated tags are not found.

This makes some workflows possible. For example, if the author loses their
repo, or switch to another computer, they can continue download their own
patches from Phabricator and update them without needing to manually create
tags.

contrib/phabricator.py

@@ -135,7 +135,9 b' def getrepophid(repo):'
     repo.ui.setconfig('phabricator', 'repophid', repophid)
     return repophid
 
-_differentialrevisionre = re.compile('\AD([1-9][0-9]*)\Z')
+_differentialrevisiontagre = re.compile('\AD([1-9][0-9]*)\Z')
+_differentialrevisiondescre = re.compile(
+    '^Differential Revision:.*D([1-9][0-9]*)$', re.M)
 
 def getmapping(ctx):
     """return (node, associated Differential Revision ID) or (None, None)
@@ -143,15 +145,26 b' def getmapping(ctx):'
     Examines all precursors and their tags. Tags with format like "D1234" are
     considered a match and the node with that tag, and the number after "D"
     (ex. 1234) will be returned.
+
+    If tags are not found, examine commit message. The "Differential Revision:"
+    line could associate this changeset to a Differential Revision.
     """
     unfi = ctx.repo().unfiltered()
     nodemap = unfi.changelog.nodemap
+
+    # Check tags like "D123"
     for n in obsolete.allprecursors(unfi.obsstore, [ctx.node()]):
         if n in nodemap:
             for tag in unfi.nodetags(n):
-                m = _differentialrevisionre.match(tag)
+                m = _differentialrevisiontagre.match(tag)
                 if m:
                     return n, int(m.group(1))
+
+    # Check commit message
+    m = _differentialrevisiondescre.search(ctx.description())
+    if m:
+        return None, int(m.group(1))
+
     return None, None
 
 def getdiff(ctx, diffopts):