Refine repo_pull.py

Use `repo manifest` command to retrieve manifest.
Change working directory to $ANDROID_BUILD_TOP before executing
commands.

Bug: 159346230
Test: ./repo_pull.py bash '<some commit query>' \
  -g https://android-review.googlesource.com
Change-Id: Ie5d29a4ae7b4b0e00c0900c8445e318dac807c77
1 file changed