util: add an mmapread method
This is useful for large files that are only partly touched.
Test Plan:
Will be used and tested in a later patch.
Differential Revision: https://phab.mercurial-scm.org/D476
# See http://EditorConfig.org for the specification
root = true
[*.py]
indent_size = 4
indent_style = space
trim_trailing_whitespace = true
[*.{c,h}]
indent_size = 8
indent_style = tab
trim_trailing_whitespace = true