fix build when git information is not available

[figsoda]

No description provided.

[Myriad-Dreamin]

It looks strange for me to retrieve git revision hash manually with having an existing well-maintained crate vergen, though laurmagedje said he don't like a things like git2rs which will introduce many build dependencies. One could use embedded git by git2 feature or git command line interface by gitcl feature.

[KillTheMule]

You can see at #292 that I tried to use one lib for this, but it triggered security checks, so I simply inlined the needed functionality. I'd still say that not using a lib here is beneficial b/c of compile time dependencies piling up, but that's certainly @nvarner 's call to make.

[nvarner]

Since it's not a major piece of functionality, and the official releases are built in an environment with git, I think this is a better solution than taking on a big dependency. It's a bit hacky, but it works. I'll try to get this merged soon.