context: move dirty() to committablectx
This is a pedantic move. It should be an error if dirty() is called on a
read-only context. Based on Mads Kiilerix's and my work at the sprint.
# 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