templater: explode if we try to emit a str
Without this if branch, we infinitely recurse in _flatten, which is
very confusing. Something in an hgweb template is trying to write out
a string instead of a bytes on Python 3, and this at least makes it
crash politely.
Differential Revision: https://phab.mercurial-scm.org/D1088
Mercurial
=========
Mercurial is a fast, easy to use, distributed revision control tool
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$ make # see install targets
$ make install # do a system-wide install
$ hg debuginstall # sanity-check setup
$ hg # see help
Running without installing::
$ make local # build for inplace usage
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See https://mercurial-scm.org/ for detailed installation
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