Mark pre-absorb commit with PRE_ABSORB_HEAD ref#168

tummychow merged 3 commits intotummychow:mastertummychow/git-absorb:masterfrom

sh-at-cs:fix-gh-157-pre-absorb-head-refsh-at-cs/git-absorb:fix-gh-157-pre-absorb-head-refCopy head branch name to clipboard