.. and also keep the history!

Step 1: use the convert extension for extraction

Step 1a: Enable Extension

Step 1b: Create a filemap

include repo_subdir
exclude repo_subdir/bigfile.csv
rename repo_subdir .

Step 1c:

hg convert --filemap /path/to/filemap /path/to/old/repo /path/to/extracted/repo

Step 2: Pull the created repo into your current repo

cd /path/to/repo
hg pull /path/to/extracted/repo