templatekw: use ctx.rev() instead of casting context to int
basectx has __int__ implemented. Per mailing list discussion
with Yuya, we don't like having this implemented because it is
too much magic and in some cases rev() will return None, which
isn't an int.
So convert a `'%d' % ctx` to ctx.rev() instead.
Differential Revision: https://phab.mercurial-scm.org/D2423
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