git source plugin: Support old style --tags option in git fetch

The --tags option before git 1.9.0 used to mean to fetch tags exclusively,
since git 1.9.0 the --tags option means to additionally fetch tags:

    https://github.com/git/git/blob/master/Documentation/RelNotes/1.9.0.txt

This patch runs git fetch twice for older versions of git, which lets us
support centos 7.
1 file changed