filemerge: support passing labels to external merge tools
This adds $labellocal, $labelother, and $labelbase to the replacement set for
merge-tools.<tool>.args config variables, and to the environment as HG_MY_LABEL,
HG_OTHER_LABEL, and HG_BASE_LABEL, respectively.
We also add merge-tools.<tool>.mergemarkers and
merge-tools.<tool>.mergemarkertemplate config variables as counterparts of
the variables available in [ui]. We are intentionally *not* respecting
ui.mergemarkers when calling out to external merge programs; too often the
default template will be too wide to display comfortably in most GUIs.
Differential Revision: https://phab.mercurial-scm.org/D2011
BasedOnStyle: LLVM
IndentWidth: 8
UseTab: ForIndentation
BreakBeforeBraces: Linux
AllowShortIfStatementsOnASingleLine: false
IndentCaseLabels: false
AllowShortBlocksOnASingleLine: false
AllowShortFunctionsOnASingleLine: false