I am having this exact issue on VMware Fusion 7.1.1 Pro. (OSX 10.10.2 guest on an OSX 10.10.2 host.)
Have shared out my top-level directory with all of my code repositories from the host, and can neither use existing repositories nor create new ones.
Trying to add a file to an existing repo from guest:
error: bad index file sha1 signature
fatal: index file corrupt
Trying to add a file to a new repo from guest:
error: insufficient permission for adding an object to repository database .git/objects
error: blah: failed to insert into database
error: unable to index file blah
fatal: adding files failed
Trying to clone a remote repo from guest:
fatal: could not create work tree dir 'github-repo'.: Input/output error
It would definitely seem that there is something going on with the shared filesystem that is wreaking havoc on file permissions as far as git is concerned. Important to note that if I do the exact same thing with an Ubuntu guest, I don't have any problems.