fix uscan with version mangling

1 job for debian/master in 6 minutes and 33 seconds (queued for 4 seconds)
Status Job ID Name Coverage
  Test
failed #153170
build

00:06:33

 
Name Stage Failure
failed
build Test
+ echo 'make debian/master branch current for gbp'
make debian/master branch current for gbp
+ git checkout -B debian/master 3ce3e199cd046e3c31fd3c953cd46fdd9836cf6a
Switched to and reset branch 'debian/master'
Your branch is up to date with 'origin/debian/master'.
+ '[' -z '' ']'
+ gbp buildpackage -uc -us --git-debian-branch=debian/master
gbp:error: v2.0_rc7 is not a valid treeish
ERROR: Job failed: exit code 1