Migration from CVS to Git looses merges

[Urs Thuermann <urs@isnogud.escape.de>]

I want to convert several old CVS repositories to Git.  Some of these
CVS repositories contain branches, which have later been merged to the
main trunk.  When I try to convert using cvs2git or git cvsimport the
branches appear in the new git repository but they are not merged to
the master branch.

Here is an example of how the branches in the CVS repository were
created and merged:

    cd /tmp
    export CVSROOT=$PWD/CVS

    cvs init
    mkdir CVS/foo
    cvs co foo

    cd foo
    (date; seq 10; date) > bar
    cvs add bar
    cvs ci -m msg1                              # rev 1.1

    sleep 1
    printf "1c\n%s\n.\nwq\n" "`date`" | ed bar
    cvs ci -m msg2                              # rev 1.2

    sleep 1
    cvs tag -b a-branch

    sleep 1
    printf "1c\n%s\n.\nwq\n" "`date`" | ed bar
    cvs ci -m msg3                              # rev 1.3

    sleep 1
    cvs up -r a-branch
    printf "12c\n%s\n.\nwq\n" "`date`" | ed bar
    cvs ci -m msg-b1                            # rev 1.2.2.1

    sleep 1
    printf "12c\n%s\n.\nwq\n" "`date`" | ed bar
    cvs ci -m msg-b2                            # rev 1.2.2.2

    sleep 1
    cvs up -A
    cvs up -j a-branch
    cvs ci -m "Merge branch a-branch"           # rev 1.4

Now I have tried 2 ways to convert this to git:

1. mkdir g; cd g; git cvsimport -A <file> -m foo
2. mkdir g; cd g; git init;
   cvs2git --blobfile=foo.blob --dumpfile=foo.dump --username=urs ../CVS/foo
   cat foo.blob foo.dump | git fast-import

In both cases, the branch "a-branch" is in the git repository but is
not merged with the master branch, i.e. rev 1.4 has only parent 1.3
but not 1.2.2.2.  I also tried cvsimport with several regexes passed
using -M to match "Merge branch a-branch", but still the same result.

How should the CVS repository be converted to git, so that the commit
corresponding to rev 1.4 has two parents, 1.3 and 1.2.2.2?

urs