.git-ftp-ignore: support / prefix

I see there are a few bug reports about how oddly the .git-ftp-ignore files work (since they match against an auto-generated temp file with multiple columns, not just the file list.)

This patch fixes up the syntax for the .git-ftp-ignore file so it's a bit more useful, and more like the .git-ignore syntax:

Lines starting with "/" match all path prefixes

Lines not starting with / match only full filenames (but with any path prefix)

It would be cool to have the same kind of globbing as .git-ignore, but this is enough of an improvement that I've been using it for months.

It's not perfect, for example you can't ignore all directories named "foo" that appear anywhere in the tree.

Just thought I'd throw this out there in case you all want to use it, or do something similar.