tests: add missing b prefix in test-context.py

# skip-blame just a b prefix

Differential Revision: https://phab.mercurial-scm.org/D3925

tests/test-context.py

@@ -77,7 +77,7 b' ctxb = context.memctx(repo, [ctxa.node()'
 print(ctxb.status(ctxa))
 
 # test performing a diff on a memctx
-diffopts = diffutil.diffallopts(repo.ui, {'git': True})
+diffopts = diffutil.diffallopts(repo.ui, {b'git': True})
 for d in ctxb.diff(ctxa, opts=diffopts):
     printb(d, end=b'')