Add parser.GitCommit field to ParseInfo struct

[stephen-soltesz]

Since #972, the ETL Version and GitCommit are compiled in at build time. And, the Version is always a human readable symbolic name; either the branch (e.g. sandbox-soltesz, master) or tag (e.g. prod-v0.6.2).

However, the ParseInfo.Version value is chosen using complex logic https://github.com/m-lab/etl/blob/master/parser/parser.go#L25 and the branch names (helpful for for quick identification) do not differentiate new builds.

We should add a new field to the ParseInfo struct for GitCommit that will be inherited by the standard column schemas used in the ndt7, annotation datatypes and simplify the selection logic in parser.go.

[laiyi-ohlsen]

Pending deployment