"git add sub1/ sub2/" doesn't work as expected when sub1 and sub2 has .git under them

Assume directories sub1 and sub2 have .git under them. "git add sub1/"
works as expected (files under sub1/ are added), howerver "git add
sub1/ sub2/" doesn't work as expeced (sub1 and sub2 are added as
submoudle).

$ git --version git version 1.6.0.4.617.g2baf1

# create repo super
$ mkdir super && cd super && git init && git commit --allow-empty -m
"Initial commit"

# create repo super/sub1 with file sub1-file
$ mkdir sub1 && cd sub1 && touch sub1-file
$ git init && git add . && git commit -m "Initial commit" && cd ..

# create repo super/sub2 with file sub2-file
$ mkdir sub2 && cd sub2 && touch sub2-file
$ git init && git add . && git commit -m "Initial commit" && cd ..

# to intend to add files under sub1 and sub2, but submodules are added
$ git add sub1/ sub2/
$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       new file:   sub1
#       new file:   sub2
#

$ git rm --cached sub1 sub2

# to intend add files under sub1, ok
$ gtad sub1/
$ git status
# On branch master
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#       new file:   sub1/sub1-file
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#       sub2/


Ping Yin