moveconfig: Allow piping in 'git show --stat' output

It is useful to be able to process only a subset of boards to save time.
Often that subset is defined by the defconfig files in a git commit. This
change allows things like:

   # Build the database
   ./tools.moveconfig.py -b

   # Find some implying configs
   ./tools/moveconfig.py -i CONFIG_X

   # Add some 'imply' statements to Kconfig files
   ./tools/moveconfig.py -i CONFIG_X -a CONFIG_A,CONFIG_B

   # Reprocess the defconfig files to see if we can drop some changes
   git show --stat | ./tools/moveconfig.py -s -d -

   # Update the commit, with fewer defconfig changes
   gii commit -au

Where the commit contains defconfig files, this will reprocess them to
take account of the imply statements that you added.

Signed-off-by: Simon Glass <sjg@chromium.org>