Embed Git commit id into `ghc --info` output

Since we switched to a Git submodule based GHC Git repo, `ghc.git`'s
commit id uniquely identifies the state of the GHC source-tree. So
having that information embedded into the `ghc` executable provides
valuable information to track accurately (especially when created by
buildbots) from which source-tree-state a given `ghc` snapshot
(distribution) was generated.

So this commit adds a new field `"Project Git commit id"` to the
`ghc --info` meta-data containing the `./configure`-time Git commit id
as reported by `git rev-parse HEAD`.

This field can also be queried with `ghc --print-project-git-commit-id`.

For source distributions, the file `GIT_COMMIT_ID` is created (with some
sanity checking to detect stale commit ids, as that would render this
information rather useless)

Reviewed By: austin

Differential Revision: https://phabricator.haskell.org/D528