Make sys_info more robust

[takluyver]

• Don't call it at import time
• Don't fail if git isn't available
• Don't use shell=True to invoke git

Closes gh-2468

[minrk]

👍, though I have a vague recollection that shell=True was needed for it to work on Windows.

[takluyver]

I dug into git history, and it appeared to have been shell=True from when it was originally written in IPython - we don't seem to have started with shell=False and then changed it in response to an issue. That doesn't guarantee that it works on Windows, and I also vaguely recall some issues with invoking other things this way, but I think it's worth a try.

[sarathuniverse]

where to change this shell=true
I mean in which file i can change

[takluyver]

This is a pull request that was already merged. You can see the files that it changed under the 'Files changed' tab at the top.