Fix for version.sh when libvpx is checked out as git submodule
Fix for version.sh when libvpx is checked out as git submodule

When using git submodules, .git may be a file instead of a directory.
The -d test was failing in that case; switched to -e.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1215

Change-Id: Iedf0e92bfeb003b28a415945dc729e6ce58c4fe4

luctrudeau/aom,GrokImageCompression/aom,mwgoldsmith/libvpx,webmproject/libvpx,mbebenita/aom,webmproject/libvpx,smarter/aom,mwgoldsmith/vpx,luctrudeau/aom,mwgoldsmith/vpx,mwgoldsmith/libvpx,mbebenita/aom,luctrudeau/aom,ShiftMediaProject/libvpx,smarter/aom,mwgoldsmith/libvpx,Topopiccione/libvpx,smarter/aom,GrokImageCompression/aom,Topopiccione/libvpx,luctrudeau/aom,mwgoldsmith/libvpx,ShiftMediaProject/libvpx,smarter/aom,webmproject/libvpx,ShiftMediaProject/libvpx,GrokImageCompression/aom,mwgoldsmith/vpx,mbebenita/aom,smarter/aom,webmproject/libvpx,mwgoldsmith/vpx,luctrudeau/aom,ShiftMediaProject/libvpx,GrokImageCompression/aom,luctrudeau/aom,mbebenita/aom,smarter/aom,webmproject/libvpx,mbebenita/aom,mbebenita/aom,mbebenita/aom,Topopiccione/libvpx,mbebenita/aom,Topopiccione/libvpx,ShiftMediaProject/libvpx,mwgoldsmith/libvpx,GrokImageCompression/aom,GrokImageCompression/aom,mwgoldsmith/libvpx,mwgoldsmith/vpx,Topopiccione/libvpx,Topopiccione/libvpx,webmproject/libvpx,mwgoldsmith/vpx,mbebenita/aom