mergestate: add `allextras()` to get all extras
`extras()` can only be used for getting extra for a file. However at
couple of places in code, we wanted to iterate over all the extras stored with
the mergestate and they were accessing the private `_stateextras`.
We add a new function for this.
Differential Revision: https://phab.mercurial-scm.org/D9190
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2