Simplify the shims registration implementation in rbenv-rehash

It doesn't need to be a bash array and we don't need a separate index of
shims registered. Simply keep everything in a space-separated string and
use that as an index as well.

This assumes that executable names *never* have spaces in them.

test/rehash.bats

@@ -53,7 +53,7 @@ ruby
 OUT
 }
 
-@test "removes stale shims" {
+@test "removes outdated shims" {
   mkdir -p "${RBENV_ROOT}/shims"
   touch "${RBENV_ROOT}/shims/oldshim1"
   chmod +x "${RBENV_ROOT}/shims/oldshim1"
@@ -67,6 +67,25 @@ OUT
   assert [ ! -e "${RBENV_ROOT}/shims/oldshim1" ]
 }
 
+@test "do exact matches when removing stale shims" {
+  create_executable "2.0" "unicorn_rails"
+  create_executable "2.0" "rspec-core"
+
+  rbenv-rehash
+
+  cp "$RBENV_ROOT"/shims/{rspec-core,rspec}
+  cp "$RBENV_ROOT"/shims/{rspec-core,rails}
+  cp "$RBENV_ROOT"/shims/{rspec-core,uni}
+  chmod +x "$RBENV_ROOT"/shims/{rspec,rails,uni}
+
+  run rbenv-rehash
+  assert_success ""
+
+  assert [ ! -e "${RBENV_ROOT}/shims/rails" ]
+  assert [ ! -e "${RBENV_ROOT}/shims/rake" ]
+  assert [ ! -e "${RBENV_ROOT}/shims/uni" ]
+}
+
 @test "binary install locations containing spaces" {
   create_executable "dirname1 p247" "ruby"
   create_executable "dirname2 preview1" "rspec"