[Gandiva][Crossbow] Revert template to have commit ids.

[asfimport]

We are dependent on the commit ids being present in the cross bow travis templates so that we can sync our builds against the same commit id that was used to create the artifacts.

So reverting back fetch-head to give back arrow-head.

Reporter: Praveen Kumar / @praveenbingo
Assignee: Praveen Kumar / @praveenbingo

PRs and other links:

• GitHub Pull Request #4738

_{Note: This issue was originally created as ARROW-5776. Please see the migration documentation for further details.}

[asfimport]

Pindikura Ravindra / @pravindra:
Issue resolved by pull request 4738
#4738