revlog: allow to pass an existing docket to `_loadindex()`
This will be when switching undelying files, for examples during censors and strip operation with rvlog-v2.
Differential Revision: https://phab.mercurial-scm.org/D10800
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2