fetch_strategy: improve GitFetchStrategy (#10153)

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
```