tests: enter full hex hash in plain text in bundle part

We were looking it up be prefix by repo.__getitem__, which I'm about
to drop support for. It's easiest to just include the full hash in
plain text.

Differential Revision: https://phab.mercurial-scm.org/D3168

tests/test-bundle2-exchange.t

@@ -787,7 +787,7 b' Check abort from mandatory pushkey'
   >     enc = pushkey.encode
   >     part = bundler.newpart('pushkey')
   >     part.addparam('namespace', enc('phases'))
-  >     part.addparam('key', enc(pushop.repo['cd010b8cd998'].hex()))
+  >     part.addparam('key', enc('cd010b8cd998f3981a5a8115f94f8da4ab506089'))
   >     part.addparam('old', enc(str(0))) # successful update
   >     part.addparam('new', enc(str(0)))
   >     def fail(pushop, exc):