How to import a exist git repository into another git repository?

Hi,

I have two exit repos A and B. I would like A becomes part of B. For example:
original:
A
|-- a.cpp
`-- Makefile

B
|-- b.cpp
`-- Makefile

combined:
B
|-- b.cpp
|-- Makefile
`-- A
    |-- a.cpp
    `-- Makefile

The git log of repo A is kept in the combined repo.

I try google but the following methods turn out they do not work like
what I want. I can not view the log of original repo A after
combining.
1) http://stackoverflow.com/questions/1683531/how-to-import-existing-git-repository-into-another
Fail at the following command.
git checkout -b ZZZ other/master
error: Untracked working tree file 'Makefile' would be overwritten by merge.

2) http://thread.gmane.org/gmane.comp.version-control.git/5126/
git log only give the lastest commit message. I do not know how to get
the log if orignal repo A.

Thank you!

Best regards,
Halley