Gracefully handle files containing an "@"

The "@" sign in file names in SVN marks the beginning of a pegged
version number -- from the Subversion book:

    Peg revisions are specified to the Subversion command-line client
    using at syntax, so called because the syntax involves appending an
    “at sign” (@) and the peg revision to the end of the path with which
    the revision is associated.

The trivial workaround is to always append an at sign to the end of the
path in the version control checks.

Before:

    $ community-stagingpkg 'Add systemd units.'
    ==> ERROR: exim-submission@.service is not under version control
    $ svn status -v | grep 'exim-submission@.service'
    A                -       ?   ?           exim-submission@.service

After:

    $ community-stagingpkg 'Add systemd units.'
    ==> Committing changes to trunk...done
    ==> Signing package exim-4.80-2-x86_64.pkg.tar.xz...
    [...]

Signed-off-by: Lukas Fleischer <archlinux@cryptocrack.de>
Signed-off-by: Pierre Schmitz <pierre@archlinux.de>