mergetool: warn if ui.merge points to nonexistent tool This adds a warning when ui.merge is configured but points to an executable that doesn't exist. It gets printed once per fail, but that seems to be how our other warnings about merge tools are reported. Differential Revision: https://phab.mercurial-scm.org/D3975

tests: demonstrate that no requested merge tool is ignored if missing If you explicitly configure a merge tool, it seems wrong that we don't even warn if we can't find it. This patch adds a test case that demonstrates that. Differential Revision: https://phab.mercurial-scm.org/D3974