Add git commit hash to bs_version.ml

[ryyppy]

I am regularly building BuckleScript JS bundles out of non-tagged revisions of BuckleScript and it's hard to keep track on what exact BS code the bundle is based on.

Is it possible to extend the build_version.js script in a way that generated following (extended) bs_version.ml file?
I'd expect a module something like this:

let version = "8.2.0"
let header = 
   "// Generated by BUCKLESCRIPT, PLEASE EDIT WITH CARE"  
let package_name = "bs-platform"   

let git_commit = "123456"

That way we can easily provide full version info in the playground and render a direct link to the exact commit as well

[bobzhang]

it should be doable, the only concern is that it would create too much noise. imagine that you have such diff every commit

[ryyppy]

Found a solution that doesn't involve any diffs / build system changes. This will be fixed by #4518