sketching the tools used

First step, is to retrieve only the commit Hashes related to this directory:

git log --format=%H -- MY_DIRECTORY

to filter out only the commit Hashes related to this directory. Then:

git format-patch HASH -1

will be used to output patch files with the commit information (subject, author...)

Then, with some sed you'll have to change the directories used in all the outputed patch:

sed -i s%MYOLDDIRECTORY%MYNEWDIRECTORY%g *.patch

The new directory in my case was "" (nothing) as it was in the root folder.

Finally, you'll have to apply your patches to your other repository with:

git am *.patch

Complete commands used

The complete commands that I've used:

cd PARENT_OF_MYDIRECTORY
git log --format=%H -- MYDIRECTORY | tac | \
    (i=0
     while read h; do
         i=$[$i+1]
         git format-patch $h -1 --start-number $i
     done)

This produces NNNN-*.patch files, one for each commit:

sed -i s%MYDIRECTORY%%g *.patch

This will update the paths in all patches:

mv *.patch MY_NEW_REPO
cd MY_NEW_REPO
git am *.patch

A quick check with gitk ensures me that message subject and commit were transfered ok.

Et voilà !