.pytools/Plugin/LicenseCheck: Use temp directory for git diff output

.pytools/Plugin/LicenseCheck: Use temp directory for git diff output

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=3746

Use --output option in git diff command to remove code diffs
from build log on stdout when LicenseCheck plugin is run.
Instead, create a temp directory for the diff output file and
remove the temp directory after the diff output is processed.

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