hg-fast-export.py: Rewrite merge logic

hg-fast-export.py: Rewrite merge logic

Merges were completely broken as they ended up with twice the same
parent in git. Still everything besides gitk seemed to work.

This because of 1) the incorrect assumption that a commit's parent is
the commit exported right before it and 2) some confusion with markes
being saved/loaded/used since git-fast-import doesn't allow for a ":0"
mark to map hg revision 1:1 to marks.

The merge "algorithm" now works like this:

1) If the commit's higher parent (highest hg rev), is not the last
commit exported for a particular branch, we issue a "from" command to
place it on top of it.

2) If the commit's lower parent exists, we issue a "merge" for it.

This is much simpler and seems to produce correct merges in git. And
while I'm at it, make output less confusing by prepending the target
branch name to each line.

The "twice the same parent" bug was discovered by Michele Ballabio on
the git list.

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