merge work from mercurial repository ./X/path/to/dir into repository ./Y/path/to/dir

gistfile1.txt

# from the bash prompt...

# starting from the directory that contains repositories X and Y, use 'hg convert --filemap' to find 
# extract the content from path/to/dir in repository X and put it into a temporary repository TMP
echo 'include "path/to/dir"' > filemap.txt
hg convert --filemap.txt ./X ./TMP

# enter repository Y and do a "force pull" to pull in the changes from the temporary repository TMP
cd ./Y
hg pull -f ../TMP

# this should create a new head because the repository histories in Y and TMP are unrelated. 
# Now just need to merge in the changes and check it all in. At this point path/to/dir should be 
# in repository Y
hg merge
hg ci -m "merging work from repository X/path/to/dir into repository Y"