Fix bug in git-daemon

The git-daemon command currently doesn't work and displays the following
error whenever a repository is cloned:

```
error: cannot run daemon: No such file or directory
[10920] unable to fork
```

On the client side the connection is simply terminated. The problem is,
that git-daemon tries to start a new instance of itself for every
new client that is connecting. It expects `argv[0]` to contain
`"git-daemon"`, but since it is converted into a builtin command, `argv[0]`
only contains `"daemon"`, which does not exist and causes the above error.
The fix simply prepends `"git"` to the list of arguments, so that the
resulting call looks something like `git daemon --serve ...`

Signed-off-by: Andreas Rohner <andreas.rohner@gmx.net>