.pytool/Plugin/UncrustifyCheck: Update func to return absolute paths

.pytool/Plugin/UncrustifyCheck: Update func to return absolute paths

Currently, UncrustifyCheck._get_git_ignored_paths() is documented to
return a list of absolute file paths but it currently returns a list
of relative file paths.

This change updates the function to return a list of absolute file
paths. The result is later compared to the list of absolute file
paths for files to run against Uncrustify.

Cc: Michael D Kinney <michael.d.kinney@intel.com>
Cc: Liming Gao <gaoliming@byosoft.com.cn>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Signed-off-by: Michael Kubacki <michael.kubacki@microsoft.com>
Reviewed-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Jiewen Yao <jiewen.yao@intel.com>