commit_id_.py is capable of generating the default commit.h.
This makes it so we always take a single path in gn.

Remove the existing commit.h and generate it into the root
generation folder (not the id subfolder) because Android blueprints
can't handle generating into subfolders that don't exist.

Make the <angle_dir> argument capable of taking a filename or
directory name. This allows us to pass the .git/HEAD file which is
a gn input.  Android blueprints require all paths used as input
or output to a script are listed as inputs or outputs in the genrule.

BUG=angleproject:2344

Change-Id: Ifd9c8331f421586db6f2c6e17faf3242376e11d0
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2070600Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>