Bash session testing the creation of a git repo and using subtree to extract the /docs dir as the gh-pages branch.

gh-pages_subtree_example.txt

waylan@dev:~$ mkdir foo
waylan@dev:~$ cd foo
waylan@dev:~/foo$ git init
Initialized empty Git repository in /home/waylan/foo/.git/
waylan@dev:~/foo$ echo "foo" > README
waylan@dev:~/foo$ git add README
waylan@dev:~/foo$ git commit -m 'Added README'
[master (root-commit) 1f31bb3] Added README
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 README
waylan@dev:~/foo$ mkdir src
waylan@dev:~/foo$ touch src/foo
waylan@dev:~/foo$ git add src/
waylan@dev:~/foo$ git commit -m "Added src/foo"
[master 40003b8] Added src/foo
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 src/foo

waylan@dev:~/foo$ mkdir docs
waylan@dev:~/foo$ touch docs/index.html
waylan@dev:~/foo$ git add docs/
waylan@dev:~/foo$ git commit -m "Added docs/"
[master 19d7b71] Added docs/
 0 files changed, 0 insertions(+), 0 deletions(-)
 create mode 100644 docs/index.html
waylan@dev:~/foo$ echo "<h1>Foo</h1>" > docs/index.html
waylan@dev:~/foo$ git add docs/
waylan@dev:~/foo$ git commit -m "Added content to docs/index.html"
[master 4b64072] Added content to docs/index.html
 1 files changed, 1 insertions(+), 0 deletions(-)
waylan@dev:~/foo$ git subtree split --branch gh-pages --prefix docs/
Created branch 'gh-pages'
ab8eb66a6770438b45505390a30232f9d50919a0
waylan@dev:~/foo$ git branch
  gh-pages
* master

waylan@dev:~/foo$ ls
docs  README  src
waylan@dev:~/foo$ git log
commit 4b6407252fc6b988de3b065bf60b211cac40cdb3
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:31:22 2012 -0500

    Added content to docs/index.html

commit 19d7b718c1e9f36495198c92bb6444560e3d375b
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:29:33 2012 -0500

    Added docs/

commit 40003b8bd67d338a4fa281f32ec74e02aa50b9b5
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:28:13 2012 -0500

    Added src/foo

commit 1f31bb3423fa1791b1a22afa4ae63844028fd26f
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:26:49 2012 -0500

    Added README
waylan@dev:~/foo$ git checkout gh-pages
Switched to branch 'gh-pages'
waylan@dev:~/foo$ ls
index.html
waylan@dev:~/foo$ git log
commit ab8eb66a6770438b45505390a30232f9d50919a0
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:31:22 2012 -0500

    Added content to docs/index.html

commit 29de5d80e1cc4c8510f8ccbb4c5c55504fe26181
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:29:33 2012 -0500

    Added docs/

waylan@dev:~/foo$ git checkout master
Switched to branch 'master'
waylan@dev:~/foo$ ls
docs  README  src
waylan@dev:~/foo$ echo "<h1>Foo</h1><p>bar</p>" > docs/index.html
waylan@dev:~/foo$ git add docs/
waylan@dev:~/foo$ git commit -m "Added more docs content."
[master 23ea643] Added morw docs content.
 1 files changed, 1 insertions(+), 1 deletions(-)
waylan@dev:~/foo$ git subtree split --branch gh-pages --prefix docs/
Updated branch 'gh-pages'
d482d07108f23728cea4fe17699d0437648c433a
waylan@dev:~/foo$ git checkout gh-pages
Switched to branch 'gh-pages'
waylan@dev:~/foo$ git log
commit d482d07108f23728cea4fe17699d0437648c433a
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:45:26 2012 -0500

    Added more docs content.

commit ab8eb66a6770438b45505390a30232f9d50919a0
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:31:22 2012 -0500

    Added content to docs/index.html

commit 29de5d80e1cc4c8510f8ccbb4c5c55504fe26181
Author: Waylan Limberg <waylan@gmail.com>
Date:   Tue Nov 6 10:29:33 2012 -0500

    Added docs/
waylan@dev:~/foo$ cat index.html
<h1>Foo</h1><p>bar</p>
waylan@dev:~/foo$ git checkout master
Switched to branch 'master'