Fix `QA::Git::Repository` so that the same `.netrc` file is not used in unrelated tests

A transient error in the Geo tests revealed a problem with how QA::Git::Repository uses .netrc.

It appends to the same file (as long as the tests run in the same process), so previous tests can affect subsequent tests. One of the Geo tests passed intermittently, but only when it ran after another test, because that other test wrote the expected credentials to .netrc.

Tests should be independent, so QA::Git::Repository should delete .netrc at the end of a test.