Show number of commits since last tag in version string

[mcwumbly (Dave McClure)]

To better distinguish between beta and tests-passed in the Admin UI:

@mcwumbly

A possible enhancement would be to show the version string as 1.0.1.beta1 + 5, where 5 is the number of commits from the one you are on since the most recent tag.

A la git describe

[zogstrip]

This is now available thanks to @riking

https://github.com/discourse/discourse/pull/3228