Git: Get commit count for specific folder

count.sh

#!/bin/bash

git log --name-only --pretty=format: -- $1 | sort | uniq -c | head -n 1

# --name-only         = Show only names of changed files
# --pretty=format:    = Remove the information, leaving only filenames
# -- $1               = Only show commits in that path (expected as argument)

# first sort will make sure things are sorted, so ...
# ... uniq -c can effectively merge all duplicates lines, counting them
# The first line is the total number of commits in that folder, retrieved by head -n 1

Faster:
git rev-list --count HEAD -- <path>

Yep, 35 vs 54 seconds counting 1.125.973 commits in the Linux kernel.