* ui/perf: do not clone same repo multiple times
Closes#2490
External repos are still created in /tmp. They might be reused until
main() end. If no perfect match is found then a repo with the same url
is copied (not cloned again) and adjusted to fit a request: rev and
cache_dir.
NOTE: test_download_error_pulling_imported_stage() was modified to not
rely on clone error.
* test: fix import update test
* test: fix external_repo test under Windows
* dvc: fix non-master default branch for external repos
* dvc: remove unused .unset_dir() method