Sync correctly when subproject url is a relative url to its parent url

Issue: when subproject url is a relative in .gitmodules
repo tool cannot handle this and cause:
"fatal: '***' does not appear to be a git repository
 fatal: Could not read from remote repository."
issue.

Signed-off-by: Shouheng Zhang <shouheng.zhang@intel.com>
Change-Id: I2a24c291ea0074ba13a740b32a11c0c25975e72b