Fix building symlinks not ending on .dsc
Commit d15d5ea6 breaks the Ubuntu lp-buildd builds. This problem can be reproduced by running:
pull-debian-source -d distro-info 1.1
mv distro-info_1.1.dsc distro-info_1.1_with_diffent_ending.txt
ln -s distro-info_1.1_with_diffent_ending.txt distro-info_1.1.dsc
sbuild -d unstable distro-info_1.1.dsc
abs_path() expands jobname from distro-info_1.1.dsc to
/full/path/to/distro-info_1.1_with_diffent_ending.txt. Then it tries
to fetch that source file (instead of using it as dsc file).
I added one test case to debian/tests/unshare. I am not sure if that's the best place to put it.
Closes: #1012856
PS: This is the last difference between the Debian and Ubuntu package of sbuild.