preindex: don't hard-code a 'v1' prefix

This task is using a different regex to the local one, and this one has
a hard-coded 'v1' prefix in it. This does not mix well with releasing
version 2.0 of git.

Copy the regex from local_index which removes the version number assumption.

This fixes #411.
Merge pull request #412 from carlosmn/cmn/preindex-v2

preindex: don't hard-code a 'v1' prefix