Shell detection: filter preceding "-"

On OSX, the shell detection code may get a preceding dash, like
"-bash". This adds a filter to remove it.

share/spack/setup-env.sh

@@ -202,7 +202,7 @@ export SPACK_ROOT=${_sp_prefix}
 # Determine which shell is being used
 #
 function _spack_determine_shell() {
-	ps -p $$ | tail -n 1 | awk '{print $4}' | xargs basename
+	ps -p $$ | tail -n 1 | awk '{print $4}' | sed 's/^-//' | xargs basename
 }
 export SPACK_SHELL=$(_spack_determine_shell)