context: add instabilities() method to basefilectx

This method is now used in webutils.commonentry(), which adds common data items
(commit hash, author, date, etc) for rendering changesets in hgweb. Usually,
commonentry() is given a changectx as ctx; but in views related to files (e.g.
file view, diff, annotate) it's replaced by a filectx, so the latter also needs
to have instabilities() method.

mercurial/context.py

@@ -821,6 +821,8 b' class basefilectx(object):'
         return self._changectx.phasestr()
     def obsolete(self):
         return self._changectx.obsolete()
+    def instabilities(self):
+        return self._changectx.instabilities()
     def manifest(self):
         return self._changectx.manifest()
     def changectx(self):

mercurial/hgweb/webutil.py

@@ -362,6 +362,7 b' def commonentry(repo, ctx):'
         'extra': ctx.extra(),
         'phase': ctx.phasestr(),
         'obsolete': ctx.obsolete(),
+        'instabilities': [{"name": i} for i in ctx.instabilities()],
         'branch': nodebranchnodefault(ctx),
         'inbranch': nodeinbranch(repo, ctx),
         'branches': nodebranchdict(repo, ctx),