Git log doesn't show submodule changes [closed]

Question

I have a commit on master, hash 6877146, which updates a submodule.

$ git show 6877146
commit 6877146f1781bfbd4ec3ae2a27121a606f5f688e
Author: [snip]
Date:   Wed Feb 22 16:10:20 2012 +0000

    updating potatobase

diff --git a/potatobase b/potatobase
index 5877e2c..b77ba62 160000
--- a/potatobase
+++ b/potatobase
@@ -1 +1 @@
-Subproject commit 5877e2c2d82645fa44f121884291ee48cf24584d
+Subproject commit b77ba624d6a1c5e62d434ad2d06383604aeab431

This commit is on the master branch, which I currently have checked out:

$ git branch -a --contains 6877146
* master
  remotes/origin/HEAD -> origin/master
  remotes/origin/master
  remotes/origin/ticket-1479-refactor-blade-json-functions

However, doing git log on the submodule does not show the commit:

$ git log potatobase | grep 6877146
$

If I explicitly check out this commit, it's in the log:

$ git checkout 6877146
$ git log potatobase | grep 6877146
commit 6877146f1781bfbd4ec3ae2a27121a606f5f688e

Why is this commit not shown in the log when I've checked out master? It's already merged into master:

$ git checkout master
$ git merge 6877146
Already up-to-date.
$

Answer 1

A commit updating a submodule just changes the revision git will checkout when you init/update the submodule. The commit id of that commit is in no way related to commit ids of the submodules itself.