git_config: fix encoding handling in GetUrlCookieFile
git_config: fix encoding handling in GetUrlCookieFile

Make sure we decode the bytes coming from the subprocess.Popen as
we're treating them as strings.

Change-Id: I44100ca5cd94f68a35d489936292eb641006edbe
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/253973
Reviewed-by: Jonathan Nieder <96e93edf89a1b75e35c5d08e01778cfc4fff73d9@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@google.com>

couchbasedeps/git-repo,lipro-yocto/git-repo,GerritCodeReview/git-repo,lewixliu/git-repo,mer-tools/git-repo,lipro-yocto/git-repo,mer-tools/git-repo,couchbasedeps/git-repo,TheQtCompany/git-repo,lewixliu/git-repo,codetutil/git-repo,TheQtCompany/git-repo,aosp-mirror/tools_repo,codetutil/git-repo,GerritCodeReview/git-repo,aosp-mirror/tools_repo