indexer: explicitly set heads/<branch>
indexer: explicitly set heads/<branch>

Without the branch set. Each git fetch will result into
fetch a full fresh git pack. This can result in running
quickly out of space.

Seen the issue on Centos 7 with git 1.8.x.

Change-Id: I56305a365fd28e8d0758d43e608e4f31ef959060

morucci/repoxplorer,morucci/repoxplorer,morucci/repoxplorer,morucci/repoxplorer