Let me copy-paste the original bug report from the Debain mail list by Paul Wise http://comments.gmane.org/gmane.linux.debian.devel.bugs.general/1321117. The bug is still reproducible with the latest version of git-prompt.sh.
I recently did a git svn clone of the Planet Debian SVN and I noticed that the normal upstream indicator in the git part of my $PS1 was not working properly. The issue appears to be that the code decides that the upstream is trunk instead of origin/trunk. trunk gives an error from git rev-list but origin/trunk does not. It works for normal git svn clone because there the remote branch is just named git-svn with no origin/ prefix. I'm using the below settings for my bash git PS1. In case you need a small repo to reproduce this with, try neomis. It still works with some of my old repos because the remote branch is just trunk instead of origin/trunk, so I think git-svn changed this.
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