Bug: partially staged file is committed in whole, if there is another staged file

If I have two modified files, and file1 is staged (git add file1), and file2 is partially staged (git add -p file2), and then try to commit only the partially staged file2, git commits *all* changes to file2, not just the staged part.

There is no bug without file1 also being staged.

Here is a transcript illustrating the problem:

$ uname -v Darwin Kernel Version 14.0.0: Tue Jul 29 16:22:25 PDT 2014; root:xnu-2782.1.62~4/RELEASE_X86_64

$ git version git version 2.1.0

$ git status -sb
## master
M  file1
MM file2
$ git commit file2 -m "adding patch"
[master 066a6e2] adding patch
 1 file changed, 2 insertions(+)

$ git status -sb ## master M file1 $

Regards, Paul