environment: make `get_git_namespace()` self-contained

environment: make `get_git_namespace()` self-contained

The logic to set up and retrieve `git_namespace` is distributed across
different functions which communicate with each other via a global
environment variable. This is rather pointless though, as the value is
always derived from an environment variable, and this environment
variable does not change after we have parsed global options.

Convert the function to be fully self-contained such that it lazily
populates once called.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>