filemerge: make capability check for internal tools ignore merge-tools section
This is follow up of 4d7b11877dd0.
Before this patch, capability check of internal merge tools falls back
to _toolbool(), which examines configurations in "merge-tools" section.
But "hg help config" explicitly says that "merge-tools" section
configures external merge tools.
Therefore, this patch makes capability check for internal tools in
hascapability() always ignore configurations in merge-tools section.
In this patch, command line configurations below are added at tests in
tests/test-merge-tools.t, in order to confirm that explicit
configuration is intentionally ignored at tool selection.
--config merge-tools.:INTERNAL_TOOL.CAPABILITY=true
{
"conduit_uri": "https://phab.mercurial-scm.org/api",
"repository.callsign": "HG",
"arc.land.onto.default": "@",
"base": "hg:.^"
}