histedit: use new function for getting first line of a string
This fixes a crash you can run into if you enter a commit message
that's just a "newline-like" byte, like a form feed byte (`hg ci -m
\x0f` in Fish). That bug is the motivation for this series.
Differential Revision: https://phab.mercurial-scm.org/D12405
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false
IncludeCategories:
- Regex: '^<'
Priority: 1
- Regex: '^"'
Priority: 2