fast-rebase: demonstrate merge-ort's API via new test-tool command

Add a new test-tool command named 'fast-rebase', which is a
super-slimmed down and nowhere near as capable version of 'git rebase'.
'test-tool fast-rebase' is not currently planned for usage in the
testsuite, but is here for two purposes:

  1) Demonstrate the desired API of merge-ort.  In particular,
     fast-rebase takes advantage of the separation of the merging
     operation from the updating of the index and working tree, to
     allow it to pick N commits, but only update the index and working
     tree once at the end.  Look for the calls to
     merge_incore_nonrecursive() and merge_switch_to_result().

  2) Provide a convenient benchmark that isn't polluted by the heavy
     disk writing and forking of unnecessary processes that comes from
     sequencer.c and merge-recursive.c.  fast-rebase is not meant to
     replace sequencer.c, just give ideas on how sequencer.c can be
     changed.  Updating sequencer.c with these goals is probably a
     large amount of work; writing a simple targeted command with
     no documentation, less-than-useful help messages, numerous
     limitations in terms of flags it can accept and situations it can
     handle, and which is flagged off from users is a much easier
     interim step.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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