convert filter: supply path to external driver

convert filter: supply path to external driver

Filtering to support keyword expansion may need the name of
the file being filtered.  In particular, to support p4 keywords
like

    $File: //depot/product/dir/script.sh $

the smudge filter needs to know the name of the file it is
smudging.

Allow "%f" in the custom filter command line specified in the
configuration.  This will be substituted by the filename
inside a single-quote pair to be passed to the shell.

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>