Decode the result of reading http.cookiefile from git config

Bug: 1066992
Change-Id: I1d4f2c8e54f717ef61ab4e630f3f66206c56047e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2132774Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>