merge: delay debug messages for merge actions
Show messages at a point where the actions have been sorted, thus preparing for
backout of
760c0d67ce5e.
This makes manifestmerge more of a silent operation, just like 'copies' is.
Indent 'preserving' messages to make them subordinate to the action logging so
they fit in the new context. (The 'preserving' messages are quite redundant and
could also be removed completely.)
ERROR: FIXME: This is a known failure of filelog.size for data starting with \1\n
OK.