FIX: fix inconsistency in head state
TEST: test to detect incorrect handling of inner merges
FIX: put initial mark
DOC: slow switch "Fastforward" to "known"

to avoid ambiguity in case ofmerges
IMPL: merge --no-ff
TRY: test with 2 steps
TEST: add the case which has been failing for merge-no-ff
TRY TEST: test for checking how handling of no-ff merges works

not working yet because build_rebase_sequence() requires reall git calls for merges
Revert "TRY TEST: test for checking how handling of no-ff merges works"

This reverts commit b0abc7fb0380429b8246b9eb7d74abaf5a6febbb.