hbisect: use a defaultdict to avoid large allocations for a large changelogs
We can avoid a SPACE(len(changelog)) allocation by using a defaultdict.
Test Plan:
python run-tests.py test-bisect*
Differential Revision: https://phab.mercurial-scm.org/D1499
# See http://EditorConfig.org for the specification
root = true
[*.py]
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indent_style = space
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[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true