templates: extract function to `stringutil` for getting first line of text
It's surprisingly hard to get the first line from a string, so let's
have our own function in `stringutil` for it.
Differential Revision: https://phab.mercurial-scm.org/D12404
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2