Speedup: Use manifest and compare checksums to detect changes

Speedup: Use manifest and compare checksums to detect changes

Instead of feeding in everything or only something and getting merges
wrong, build up a list of changed (incl. added) and deleted files by

  1) comparing manifest (deleted, added)
  2) comparing checksums if file is present in parent and child (change)

The hg-crew and mutt imports now go in <15 minutes and md5 sums match.

Thanks to Theodore Tso for the hint.

While at it, fix a regression that upon incremental import start we
always merged a branch plus initializing it. A single test showed that
the new detection get starting off from a merge commit right, too.

Signed-off-by: Rocco Rutte <pdmef@gmx.net>