fatal: unable to read <hash>

Hi,

on one of my trees I am getting:

--- 8< 8< 8< -----------------------------------------------------------
% git fetch freebsd
Auto packing the repository in background for optimum performance.
See "git help gc" for manual housekeeping.
warning: The last gc run reported the following. Please correct the root cause
and remove .git/gc.log
Automatic cleanup will not be performed until the file is removed.

fatal: unable to read b9cdc058e6eb1b3d8b5e29ad9b911d3da98f65a7
fatal: failed to run repack
--- 8< 8< 8< -----------------------------------------------------------

I am not concerned about the background gc but the fatal error about the
hash displayed.

I have no clue how to solve this, nor do I know what object that hash
is/was.

Does anyone know how to "repair" this wihtout having to blow away the
local copy with all its development branches (which would be rather
painful)?

/bz

-- 
Bjoern A. Zeeb                                                     r15:7