Bug 1268573 - Pin revision in hgwatchman repo; r=smacleod

Per
https://www.mercurial-scm.org/pipermail/mercurial-devel/2016-April/083649.html
the hgwatchman repo has been blanked. We should pin to the last
changeset before the blanking so people can still run hgwatchman.

We'll also need to teach `mach mercurial-setup` to install the
fsnotifier extension. That's for a different patch and bug.

DONTBUILD (NPOTB)

MozReview-Commit-ID: GyuDMTwARqo

--HG--
extra : rebase_source : be782f0262006a29f5b48add98fddda9dd2cdbd7

tools/mercurial/hgsetup/update.py

@@ -39,7 +39,7 @@ class MercurialUpdater(object):
             hg,
             'https://bitbucket.org/facebook/hgwatchman',
             self.hgwatchman_dir,
-            'default',
+            '5ca0f920df7ec8a93d322f06d554f778184cdcd1',
             'Ensuring hgwatchman is up to date...')
         if repo_existed:
             print(FINISHED)