meson: bring MESA_GIT_SHA1 in line with other build systems

meson: bring MESA_GIT_SHA1 in line with other build systems

Meson's vcs_tag() uses the output of `git describe`, eg.
  17.3-branchpoint-5-gfbf29c3cd15ae831e249+

Whereas the other build systems used a script that outputs only the sha1
of the HEAD commit, eg.
  fbf29c3cd1

Given that this information is used by printing it next to the version
number, there's some redundancy here, and inconsistency between build
systems.

Bring Meson in line by making it use the same script, with the added
advantage of now supporting the MESA_GIT_SHA1_OVERRIDE env var.

Signed-off-by: Eric Engestrom <eric@engestrom.ch>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>