This allows us to delete the Windows batch file.

Changes the commit_id script to take the working directory so that it
can be called from a different working directory than the angle
repository is in.

Renames the generated commit header to angle_commit.h. This is being
written to the shared generated code directory for the entire build,
and "commit.h" is insufficiently unique.

BUG=angle:669

Change-Id: I35e80411a7e8ba1e02ce3f6a4fc54ed4dbc918f3
Reviewed-on: https://chromium-review.googlesource.com/202048Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>