Currently, only the Git repsository's URL shows up in the `spack info`
output, which makes it hard to distinguish different versions:
```
Safe versions:
develop [git] https://github.com/LLNL/H5Z-ZFP.git
0.8.0 [git] https://github.com/LLNL/H5Z-ZFP.git
0.7.0 [git] https://github.com/LLNL/H5Z-ZFP.git
```
This change adds additional information as shown when cloning a Git
repository:
```
Safe versions:
develop [git] https://github.com/LLNL/H5Z-ZFP.git on branch master
0.8.0 [git] https://github.com/LLNL/H5Z-ZFP.git at commit af165c4
0.7.0 [git] https://github.com/LLNL/H5Z-ZFP.git at commit 58ac811
```