setup: drop static `cwd` variable

setup: drop static `cwd` variable

The current working directory is stored as part of a static strbuf
variable. This variable had to have a lifetime longer than its
containing function because the value we return typically points into
that buffer.

In the preceding commit we have moved the prefix into the repository
though. Consequently, we can now return the repository's prefix instead
of the local one and thus properly manage the lifecycle of this local
variable.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>