tools/git-sync-deps improvements

tools/git-sync-deps improvements

  * Use `/usr/bin/env python` patten: more robust on non-standard systems.
  * Factor out status line, make formatting clearer.
  * Alwyas call `git remote set-url origin $repo` since it is quick.
  * Find `fetch-gn` script more robustly.
  * `--help` works again.
  * handling deps_os better
  * check to see that directories don't include each other

Change-Id: I06806226e2c263147723c6326c09c5e385abc68d
Reviewed-on: https://skia-review.googlesource.com/7646
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Hal Canary <halcanary@google.com>