ShellPkg: fix Comp command output

The comp command was potentially displaying the incorrect offset into the file due to comparing 3 extra bytes for difference groups.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <Jaben.carsey@intel.com>
Reviewed-by: Tapan Shah <tapandshah@hp.com>
Reviewed-by: Erik Bjorge <erik.c.bjorge@intel.com>


git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@15558 6f19259b-4bc3-4df7-8a09-765794883524
Merge branch 'master' of https://github.com/tianocore/edk2