update-dockapps.pl: Avoid ambiguous argument error.

It is possible that, when running update-dockapps.pl, a filename may match a
revision, giving an error, e.g.,
fatal: ambiguous argument 'wmbattery': both revision and filename
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

This patch fixes this problem by inserting "--" where needed.