ShellPkg: Fix file size error upon copy operation.

ShellPkg: Fix file size error upon copy operation.

There was a case where an copy operation of a small file overwriting a larger file would not correctly remove the extra space in the old file.  The resultant file would have the entire source file and then what remained of the original file.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Jaben Carsey <Jaben.carsey@intel.com>
reviewed-by: El-Haj-Mahmoud, Samer <samer.el-haj-mahmoud@hp.com>

git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14584 6f19259b-4bc3-4df7-8a09-765794883524