filemerge: make `_filemerge()` do both premerge and merge
This patch removes the `premerge` argument from `_filemerge()` and
makes it do both the "premerge" and "merge" steps without the caller
having to call it twice.
Differential Revision: https://phab.mercurial-scm.org/D11859
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2