gitk: Use git-rev-parse only to identify file/dir names on cmd line

This uses git-rev-parse --no-revs --no-flags to give us just the
file and directory names on the command line, so that we can create
the "Command line" view if any were specified.  All other arguments
just get passed to git-rev-list (without a pass through git-rev-parse).

Signed-off-by: Paul Mackerras <paulus@samba.org>