plumbing: format/packfile, fix panic retrieving object hash.#744

mcuadros merged 2 commits intosrc-d:mastersrc-d/go-git:masterfrom

jfontan:fix/make-sure-original-data-is-saved-before-creanupjfontan/old-go-git:fix/make-sure-original-data-is-saved-before-creanupCopy head branch name to clipboard