Add --root option

[xixixao]

To consider all commits including the first commit.

[tummychow]

keep in mind that you cannot absorb through merges, so if any merge exists anywhere in the ancestry of HEAD, then you can't actually go all the way to the first commit

[yairfine]

+1 for --root, could be useful for linear rebase workflow.
BTW, why isn't it possible to absorb through merges? As merges could be rebased later with --rebase-merges.

[blairconrad]

BTW, why isn't it possible to absorb through merges? As merges could be rebased later with --rebase-merges.

See notes in #134.