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 <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>
(cherry picked from commit ded477dbb9c6993cbe8b93b10654682b04fdeea8)

git_config.py

@@ -528,7 +528,7 @@ def GetUrlCookieFile(url, quiet):
         cookiefile = None
         proxy = None
         for line in p.stdout:
-          line = line.strip()
+          line = line.strip().decode('utf-8')
           if line.startswith(cookieprefix):
             cookiefile = os.path.expanduser(line[len(cookieprefix):])
           if line.startswith(proxyprefix):
@@ -540,7 +540,7 @@ def GetUrlCookieFile(url, quiet):
       finally:
         p.stdin.close()
         if p.wait():
-          err_msg = p.stderr.read()
+          err_msg = p.stderr.read().decode('utf-8')
           if ' -print_config' in err_msg:
             pass  # Persistent proxy doesn't support -print_config.
           elif not quiet: