Trying out the fossil export to git, git complains about the format, running this shell script allows to reproduce the problem:
<code>
<pre>
--- cut here ---

PATH=`pwd`:$PATH
fossil version
git --version
fossil clone mongrel2.fossil
rm -rf mongrel2-git
mkdir mongrel2-git
cd mongrel2-git
git init
fossil export --git ../mongrel2.fossil | git fast-import
--- end cut here ---

outputs on my end:

./runme
This is fossil version [0018d724b3] 2010-12-07 13:31:37 UTC
git version
                Bytes      Cards  Artifacts     Deltas
Sent:              53          1          0          0
Received:      224521       4880          0          0
Sent:           10048        226          0          0
Received:      472854        294         34        166
Sent:           18853        415          0          0
Received:      228964        480         23        377
Sent:           37653        801          0          0
Received:      507401        880        149        651
Sent:           75253       1601          0          0
Received:     2407750       1680        548       1052
Sent:           88366       1880          0          0
Received:     3616447       1959        584       1295
Sent:              53          1          0          0
Received:      224521       4880          0          0
Total network traffic: 123013 bytes sent, 2461523 bytes received
Rebuilding repository meta-data...
  100.0% complete...
project-id:
server-id:
admin-user: dalien (password is "xxxxxx")
Initialized empty Git repository in
fatal: Not a blob (actually a commit): M 100644 :27 src/manifest
fast-import: dumping crash report to .git/fast_import_crash_24011

</pre>
</code>