Use -- to treat filenames as non-option args to gpg.

Ordinarily I'd also check that the filename was not "-", or perhaps
get the absolute path, but in this case I know the filename. (Otherwise
this could result in a hung process as gpg waited for stdin.)
Found a couple more shell invocations.

/usr/bin/java doesn't appear to have a -- equivalent, unfortunately.
Merge pull request #985 from timmc/explicit-gpg-args

Use -- to treat filenames as non-option args to gpg.