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