download_and_compile.sh: more reliable test in _gitDownload()

Before this change, using OSG with download_and_compile.sh several times
in the same directory would fail because the upstream repo for OSG has
none of README, README.txt and README.rst at top-level (it has a
README.md). Testing for the .git dir is definitely more reliable.

download_and_compile.sh

@@ -319,11 +319,11 @@ function _gitDownload(){
   local component="$1"
   local clone_arg
 
-  if [ "$DOWNLOAD" != "y" ]; then
+  if [[ "$DOWNLOAD" != "y" ]]; then
     return
   fi
 
-  if [ -f "README" -o -f "README.txt" -o -f "README.rst" ]; then
+  if [[ -d ".git" ]]; then
     _printLog "$component: the repository already exists"
   else
     proto_spec=$(_gitProtoSpec "${REPO_PROTO[$component]}" \