How do I 'git diff' on a certain directory?

Provide a path (myfolder in this case) and just run:

git diff myfolder/

Not only you can add a path, but you can add git diff --relative to get result relative to that folder.

git -C a/folder diff --relative

And with Git 2.28 (Q3 2020), the commands in the "diff" family learned to honor the "diff.relative" configuration variable.

See commit c28ded8 (22 May 2020) by Laurent Arnoud (spk).
^{(Merged by Junio C Hamano -- gitster -- in commit e34df9a, 02 Jun 2020)}

diff: add config option relative

^{Signed-off-by: Laurent Arnoud
Acked-by: Đoàn Trần Công Danh}

The diff.relative boolean option set to true shows only changes in the current directory/value specified by the path argument of the relative option and shows pathnames relative to the aforementioned directory.

Teach --no-relative to override earlier --relative

Add for git-format-patch(1) options documentation --relative and --no-relative

The documentation now includes:

diff.relative:

If set to 'true', 'git diff' does not show changes outside of the directory and show pathnames relative to the current directory.

---

Warning: Before Git 2.34 (Q4 2021), "git diff --relative"^(man) segfaulted and/or produced incorrect result when there are unmerged paths.

See commit 8174627 (22 Aug 2021) by Đoàn Trần Công Danh (sgn).
^{(Merged by Junio C Hamano -- gitster -- in commit c8f4916, 08 Sep 2021)}

diff-lib: ignore paths that are outside $cwd if --relative asked

^{Reported-by: Thomas De Zeeuw
Tested-by: Carlo Arenas
Signed-off-by: Đoàn Trần Công Danh}

For diff family commands, we can tell them to exclude changes outside of some directories if --relative is requested.

In diff_unmerge(), NULL will be returned if the requested path is outside of the interesting directories, thus we'll run into NULL pointer dereference in run_diff_files when trying to dereference its return value.

Checking for return value of diff_unmerge before dereferencing is not sufficient, though.
Since, diff engine will try to work on such pathspec later.

Let's not run diff on those unintesting entries, instead.
As a side effect, by skipping like that, we can save some CPU cycles.

What I was looking for was this:

git diff <ref1>..<ref2> <dirname>

If you're comparing different branches, you need to use -- to separate a Git revision from a filesystem path. For example, with two local branches, master and bryan-working:

$ git diff master -- AFolderOfCode/ bryan-working -- AFolderOfCode/

Or from a local branch to a remote:

$ git diff master -- AFolderOfCode/ origin/master -- AFolderOfCode/