Push fails when --if-changed flag is provided using a zip file as the source.

[rsethc]

Using the --if-changed flag against a zip archive causes Butler to fail, whereas simply omitting the flag results in a successful push. While I suspect it's specific to zip archives (due to the "lstat ...: not a directory" text) I have not done further testing to prove that. This particular zip archive contains, at the top level, no files and one folder 'built' which contains game files within it (I can provide the archive itself, if this would help).

> Comparing against previous build...
checking for differences: lstat game.zip/built: not a directory
The command '/bin/sh -c butler push game.zip glassblade/kringle-logistics:windows --if-changed' returned a non-zero code: 1

Perhaps the error text itself is originating from here:

butler/cmd/push/push.go

Line 195 in cdcf419

return errors.Wrap(err, "checking for differences")

[bend-n]

can confirm, had to create my own diffing system (there were issues with directorys for me)