http://superuser.com/questions/586067/git-fatal-pack-has-1-unresolved-delta-index-pack-failed suggests that you might have shallow clone enabled and somehow the command line git process is unable to resolve the deltas.
If you wipe the workspace, does the job then run successfully?
Are you using a reference repository, and was that reference repository deleted or damaged in some way? If the original clone used a reference repository and the reference repository was garbage collected, it is possible that some of the content referenced from your Jenkins job was deleted from the reference repository.
Mark Waite