git log -S problem

There is something odd with -S of git log. Try this in your git.git:

$ git log --pretty=oneline -Sbuiltin-merge-base -- Makefile 
71dfbf224ff980f4085f75868dc409118418731e Make merge-base a built-in.

$ git log --pretty=oneline -Smerge-base -- Makefile
e468305a954d95a26bfcdec3bc6e4bd477d95676 [PATCH] Remove the explicit ...
a3df180138b85a603656582bde6df757095618cf Rename git core commands ...
cef661fc799a3a13ffdea4a3f69f1acd295de53d Add support for alternate ...
e590d694ead8d50c2afc7086161d4ddc5d907655 Add more header dependencies.
6683463ed6b2da9eed309c305806f9393d1ae728 Do a very simple "merge-base"...

$ git version
git version 1.5.2

I had expected that the set of commits found by the second search string are a 
proper superset of those found by the first one. What's wrong here? Why does 
a search for 'merge-base' not find occurences of 'builtin-merge-base'?

-- Hannes