What does the -r option of Git log do?

Question

The man page git-log(1) says:

-r
Show recursive diffs.

So we can put this question in another way:

What is meant by "recursive diffs" in this context.

After getting this answer from @phd, I did some tests that aim in the indicated direction:

If I execute the following commands in a current version of the Git source repository (recently cloned from https://github.com/git/git.git) with checked out master (currently pointing to commit 6a6c0f1), they give identical output:

git log --name-only -m
git log --name-only -m -r

(For this to work, one might have to increase the diff.renameLimit to round about 3150.) I tried this with Git versions 2.10.2 and 2.17.1 obtaining equal results.

In addition, the command

git log --name-only -m master~..master

outputs

commit 6a6c0f10a70a6eb101c213b09ae82a9cad252743
Author: Junio C Hamano <[email protected]>
Date:   Thu May 9 00:37:54 2019 +0900

    The eighth batch

    Signed-off-by: Junio C Hamano <[email protected]>

Documentation/RelNotes/2.22.0.txt

from the last line of which one can see that this command looks into subdirectories even without -r.

Answer 1

The option comes from git diff-tree docs and in git diff-tree it really works; see this example:

$ git diff-tree master~ master
:100644 100644 a2be0e5e5959396fb85445cfff714d7b04a1231b 5d7a2a0a4fd6fbaf439aa08bb7f17052a65a5236 M  ANNOUNCE
:100644 100644 22e746900ab77a79b4cb6780f536a517771fe276 f2aae0e9c7c87226f4f5c06ca006f4d04ce79dc1 M  ChangeLog
:040000 040000 54ef4d981c00162085347031d31286d630258153 76d21516ed54422a8981a31a9a0bf47dc5e5af6f M  mimedecode

$ git diff-tree -r master~ master
:100644 100644 a2be0e5e5959396fb85445cfff714d7b04a1231b 5d7a2a0a4fd6fbaf439aa08bb7f17052a65a5236 M  ANNOUNCE
:100644 100644 22e746900ab77a79b4cb6780f536a517771fe276 f2aae0e9c7c87226f4f5c06ca006f4d04ce79dc1 M  ChangeLog
:100644 100644 4ca2f25d2d061dba16294d67ab8018ea00be2b37 ead1ab38493c7e5119d8204a8731747cc534647c M  mimedecode/mimedecode.py

The option is included in docs at git-diff-tree.txt and the file is included in git-log.txt via diff-options.txt.

I suspect the inclusion of the option in git log docs is an artifact of doc generation. At least I cannot make the option works for me in git log.