configure: relax check for .git
configure: relax check for .git

For a top-level non-bare repository .git will be a directory,
but for submodules .git is a regular textfile containing the relative
path to the actual git-directory which will be subdir inside the
toplevel repositories git-directory.

astiob/libass,astiob/libass,libass/libass,MrSmile/libass,MrSmile/libass,ShiftMediaProject/libass,libass/libass,ShiftMediaProject/libass