index: use an explicit constant for INDEX_HEADER format and use it for docket
This avoid leaking python-3.6 compatibility details too much.
Differential Revision: https://phab.mercurial-scm.org/D10831
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2