test: also deactivate `ui.merge` when testing merge tool We are about to introduce a default config for `ui.merge`. Since `test-merge-tools.t` is about testing merge configuration, we need to make sure it won't interfere.

tests: adjust merge tool config in test-largefiles-update.t Previously, the `ui.merge=internal:fail` configuration was shadowed by the the `HGMERGE=internal:merge` environment variable. We would like to remove this usage of `HGMERGE` variable because it is error-prone (eg: this very test). We start by cleaning up test independently to avoid large churn when the change happens.

extensions: include current version in "invalid version" message It is "not so rare" for the mercurial version to be badly detected at build time. In such case, version check for extensions gets confused. To help pinpoint the error, we now include the Mercurial's version number in the error message.