Add support for git repos in a RPM source
Add support for git repos in a RPM source

This commit adds support for providing a verified git tag as a
source for an RPM.

The line in the source file should have a format similar to the following:

{"type": "git", \
 "url": "https://github.com/opensciencegrid/cvmfs-config-osg.git", \
 "tag": "v0.1",
 "hash": "e2b54cd1b94c9e3eaee079490c9d85f193c52249"}

(should be on the same line in the file; line continuations added for
readability.)

This results in a tarball named cvmfs-config-osg-v0.1.tar.gz that:
  - Contents are prefixed with cvmfs-config-osg-v0.1/
  - Contents correspond to the git tag v0.1
  - The tag v0.1 is verified to be the same as hash
    e2b54cd1b94c9e3eaee079490c9d85f193c52249 in the repository.

This provides end-to-end verification of the contents of the tarball.

opensciencegrid/osg-build,matyasselmeci/osg-build,opensciencegrid/osg-build,matyasselmeci/osg-build