git stash push -u always warns "pathspec '...' did not match any files"

[Marc Strapetz <marc.strapetz@xxxxxxxxxxx>]

Reproducible in a test repository with following steps:

$ touch untracked
$ git stash push -u -- untracked
Saved working directory and index state WIP on master: 0096475 init
fatal: pathspec 'untracked' did not match any files
error: unrecognized input

The file is stashed correctly, though.

Tested with Git 2.16.2 on Linux and Windows.

-Marc