7984: Improve version display r=matklad a=lnicola
Maybe closes#7854
The version string for unreleased builds looks like this now:
```
$ rust-analyzer --version
rust-analyzer 2021-03-08-159-gc0459c535
```
Release builds should only have the tag name (`2021-03-15`).
Co-authored-by: Laurențiu Nicola <lnicola@dend.ro>