Fix build when .git is a gitlink file

Fix build when .git is a gitlink file

If ${CMAKE_SOURCE_DIR}/.git exists, then
framework/qphelper/CMakeLists.txt uses the git revision as the dEQP
revision. The CMake rules update the revision when the files
${CMAKE_SOURCE_DIR}/.git/HEAD and ${CMAKE_SOURCE_DIR}/.git/index
change.

However, Git does not require that .git be a directory. If
git-submodules or git-worktrees are used, then .git may be a gitlink
file instead and the real git directory is elsewhere.

Fix the build by using `git rev-parse --git-dir` to find the real git dir
and `git rev-parse HEAD` to query the commit sha1.

This bumps the CMake requirement from 2.8 to 2.8.2. But that should be
safe since 2.8.2 was released in 2010.

Components: Framework
Change-Id: I811630e5afada8e6aed611b0ef6c51e39be2200b