Fix build-sdl.sh script error

[meeq]

The SDL build script fails:

./thirdparty/SDL/build-sdl.sh 

Cloning into 'SDL'...
remote: Enumerating objects: 193100, done.
remote: Counting objects: 100% (48982/48982), done.
remote: Compressing objects: 100% (1282/1282), done.
remote: Total 193100 (delta 47990), reused 47700 (delta 47700), pack-reused 144118 (from 1)
Receiving objects: 100% (193100/193100), 102.69 MiB | 15.18 MiB/s, done.
Resolving deltas: 100% (147983/147983), done.
cat: HEAD: No such file or directory
fatal: ambiguous argument '': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

This change uses the proper git command to get the commit ID for HEAD. Tested successfully.

[meeq]

Closing this PR; this fix is not quite right, and does not fully resolve the issue.

[jcm93]

This script is also only relevant for the legacy build system and faces imminent removal.