exthelper: reintroduce the ability to register templates

Authored by mharbison72.

Description

exthelper: reintroduce the ability to register templates

Same mechanism as revsets earlier in this series. The LFS extension is updated
to provide test coverage.

This also seems to make the test failure around issue6033 mentioned in
0a7f582f6f1f much less regular on Windows.

Details

Committed
mharbison72Dec 27 2018, 9:55 PM
Parents
rHG28a4fb793ba1: extensions: deprecate extsetup without a `ui` argument (API)
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