Older .gclient files might have explicit u'' strings in their
specs; as part of upgrading code to Python3 compatibility,
we apparently recently introduced a bug in the handling of this
that might cause them to fail on Windows.

This CL fixes that issue by ensuring that the GIT_DIR env var
is always set to a regular string, regardless of whether
the solution name was specified to be a regular string
or a unicode string.

Bug: 1016599
Change-Id: I295d220559eac20a2ced876672d5ccdb787a5338
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1874704Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Dirk Pranke <dpranke@chromium.org>