fix git version fail when head is detached

[jambun]

git symbolic-ref -q --short HEAD gives a fatal error on detached HEAD.

so let's try a little harder to get something useful - by giving the checked out tag.

[mark-cooper]

Hi James, depending on what's going on this can still turn up empty:

git describe --tags --exact-match 
fatal: no tag exactly matches '784cfaf6a4345dcd51aef68ad74180b309348c93'

git describe --debug 
describe HEAD
No exact match on refs or tags, searching to describe ...

So rather than nothing maybe as a final fallback use the abbrev. commit hash?

git symbolic-ref -q --short HEAD || git describe --tags --exact-match 2> /dev/null || git rev-parse -q --short HEAD
784cfaf6a

[jambun]

Sounds good @mark-cooper - feel free to do what you will with this.