`deleteRemote()` intends to return successfully if
the to-be-deleted remote does not exist. In this case, git returns an
exit code of 2, and outputs a message to stderr. Depending on the locale
of the user's system, this error message might be localized.
`spawnPromise()` and `spawnStream()` try to set the locale to `en_US`,
but there are no guarantees that this locale is actually available on
the user's system.

Instead of doing fragile locale-dependent string parsing, simply use the
exit code we get from git in this case and act on that.

Fix the mocks in the tests to behave like the real-world git, and add a
test for the non-English case as well.