testing: add a utility function to wait for file create
This is similar to `tests/testlib/wait-on-file`, but for the python code
Differential Revision: https://phab.mercurial-scm.org/D10476
# See http://EditorConfig.org for the specification
root = true
[*.py]
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indent_style = space
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end_of_line = lf
[*.{c,h}]
indent_size = 8
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[*.t]
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