RepoCommand: Preserve executable bit in <copyfile>

RepoCommand: Preserve executable bit in <copyfile>

The copyfile entry in the manifest file copies the contents of the file
but doesn't keep the executable flag. This is inconsistent with repo
tool behaviour, plus is natural to expect that the copy of a executable
file is executable.

Transfer the executable bit when copying the file, aligning the
RepoCommand with repo tool and user expectations.

Change-Id: I01b24f482d5939e01d496f032388b3a5c02a912a
Signed-off-by: Ivan Frade <ifrade@google.com>