git failing to create new branches, depending on the name

One of my build scripts, which takes a 'work' git tree, clones it to make a build tree, then creates an unique branch there and goes on to do the rest of the work, started mysteriously failing recently . Turns out git doesn't like the branch names that my script is choosing and git-checkout fails with a misleading fatal error. I don't remember changing the script recently and it used to work for months, something must have changed.

This is what i did while trying to find why the checkout is failing:

+ git --version
git version 1.6.4
+ rm -rf /tmp/build-tree
+ git clone -l -s -n -o askern --reference /tmp/work-tree /tmp/work-tree /tmp/build-tree
Initialized empty Git repository in /tmp/build-tree/.git/
+ cd /tmp/build-tree
+ git branch -a | grep g90bc1a6 
+ git checkout -f -b branch-g90bc1a6 askern/release
fatal: git checkout: branch branch-g90bc1a6 already exists
# WTF?
+ (
+  cd /tmp/work-tree
+  git branch -a | grep g90bc1a6
+ )
# nothing in parent repo either.
+ git checkout -f branch-g90bc1a6 
Checking out files: 100% (29109/29109), done.
Note: moving to 'branch-g90bc1a6' which isn't a local branch
If you want to create a new branch from this checkout, you may do so
(now or later) by using -b with the checkout command again. Example:
  git checkout -b <new_branch_name>
HEAD is now at 90bc1a6... Merge branch 'upstream' of git://ftp.linux-mips.org/pub/scm/upstream-linus
# so where is coming from?
+ git branch -a | grep g90bc1a6
+ git status
# Not currently on any branch.
nothing to commit (working directory clean)
+  git show --pretty=short 90bc1a6 | cat
commit 90bc1a658a53f8832ee799685703977a450e5af9
Merge: 6ce90c4 54822de
Author: Linus Torvalds <torvalds@linux-foundation.org>

    Merge branch 'upstream' of git://ftp.linux-mips.org/pub/scm/upstream-linus
# Aha

So what appears to be happening is that the script picks up the 90bc1a6 suffix (it gets it from git-describe), uses it as part of the branch name and then git misinterprets it. (the actual names used by the script are much longer, i used "branch-g90bc1a6" only to figure what was going on)

artur