.pytool/Plugin/EccCheck: Remove RevertCode()

.pytool/Plugin/EccCheck: Remove RevertCode()

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2986

The RevertCode() method uses git reset which can remove
local changes. Instead of modifying the local files, a
copy of the package passed into the EccCheck tool is
copied to a temp directory in Build/ecctemp.  This same
temp directory is also used for exception.xml. The working
directory used by ECC is also set to this same temp
directory. The combination of these changes eliminates
operations that that modified the git state.

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: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Liming Gao <gaoliming@byosoft.com.cn>