[hp-diff-files] Implement hands-on T4L3/hp-diff-files (#198)

# Exercise Review

## Exercise Discussion

Fixes #197 

## Checklist

- [ ] If you require a new remote repository on the `Git-Mastery`
organization, have you [created a
request](https://github.com/git-mastery/exercises/issues/new?template=request_exercise_repository.md)
for it?
- [ ] Have you written unit tests using
[`repo-smith`](https://github.com/git-mastery/repo-smith) to validate
the exercise grading scheme?
- [X] Have you tested the download script using `test-download.sh`?
- [X] Have you verified that this exercise does not already exist or is
not currently in review?
- [ ] Did you introduce a new grading mechanism that should belong to
[`git-autograder`](https://github.com/git-mastery/git-autograder)?
- [ ] Did you introduce a new dependency that should belong to
[`app`](https://github.com/git-mastery/app)?

Author: jovnc

hands_on/diff_files.py

@@ -0,0 +1,68 @@
+import os
+
+from exercise_utils.file import create_or_update_file, append_to_file
+from exercise_utils.git import add, init, commit
+
+__requires_git__ = True
+__requires_github__ = False
+
+
+def download(verbose: bool):
+    os.makedirs("employees")
+    os.chdir("employees")
+    init(verbose)
+
+    create_or_update_file(
+        "list.txt",
+        """
+        Andy Bernard
+        """,
+    )
+    os.makedirs("andy")
+    create_or_update_file(
+        "andy/history.txt",
+        """
+        Previously in Stamford branch
+        """,
+    )
+    add(["."], verbose)
+    commit("Add Andy", verbose)
+
+    append_to_file(
+        "list.txt",
+        """
+        Pam Beesly
+        """,
+    )
+    add(["."], verbose)
+    commit("Add Pam", verbose)
+
+    append_to_file(
+        "list.txt",
+        """
+        Kelly Kapoor
+        """,
+    )
+    add(["."], verbose)
+    commit("Add Kelly", verbose)
+
+    append_to_file(
+        "list.txt",
+        """
+        Kevin Malone
+        """,
+    )
+    add(["."], verbose)
+
+    append_to_file(
+        "list.txt",
+        """
+        Jim Halpert
+        """,
+    )
+    append_to_file(
+        "andy/history.txt",
+        """
+        Education: Cornell
+        """,
+    )