.git inside a .git: is it safe?

Hi,

I'm actually working on the packaging of a project. To do so, I use
two Git repositories:
- one to follow the main project
- one to manage my updates on this project.

But, the matter is I cannot merge the both root. I need to have the main project sources in a subdirectory of my own project. My workspace is as follow: myproject/sub/tree/mainproject

With SVN, I think such things are handled with svn:external.

Actually, with Git, I frequently copy the mainproject inside its subtree in my workspace.

But, I recently discover that everything seems fine if I include the
Git repo of mainproject inside the Git repo of my project. The layout
lloks like this:
myproject
|- .git
|- sub
  |- tree
    |- mainproject
      |- .git

It seems to work. But do you think such layout is "safe" ?