Support GEM_HOME, add limited support for user-installed gems (#1436)

The rehash process will now discover executables in additional locations:
- `~/.gem/ruby/<version>/bin/*`
- `$GEM_HOME/bin`

The `rbenv which` (and thus `rbenv exec`) command will also search these locations when looking up a command. This enables shims to dispatch calls to executables added by `gem install --user-install`.

Note that this support is limited:
- It will only work with C Ruby, as it's difficult to guess the `~/.gem/<engine>/<version>` directory for other Rubies without actually loading Ruby;
- It will only work for RBENV_VERSION values in the format `X.Y.Z` and not "system".

test/rehash.bats

@@ -3,10 +3,11 @@
 load test_helper
 
 create_executable() {
-  local bin="${RBENV_ROOT}/versions/${1}/bin"
-  mkdir -p "$bin"
-  touch "${bin}/$2"
-  chmod +x "${bin}/$2"
+  local exe="${RBENV_ROOT}/versions/${1}/bin/${2}"
+  [ -n "$2" ] || exe="$1"
+  mkdir -p "${exe%/*}"
+  touch "$exe"
+  chmod +x "$exe"
 }
 
 @test "empty rehash" {
@@ -104,6 +105,42 @@ ruby
 OUT
 }
 
+@test "user-install" {
+  create_executable "${HOME}/.gem/ruby/3.0.0/bin/lolcat"
+  create_executable "${HOME}/.gem/ruby/3.1.0/bin/pinecone"
+
+  assert [ ! -e "${RBENV_ROOT}/shims/lolcat" ]
+  assert [ ! -e "${RBENV_ROOT}/shims/pinecone" ]
+
+  run rbenv-rehash
+  assert_success ""
+
+  run ls "${RBENV_ROOT}/shims"
+  assert_success
+  assert_output <<OUT
+lolcat
+pinecone
+OUT
+}
+
+@test "explicit gem home" {
+  create_executable "${HOME}/mygems/bin/lolcat"
+  create_executable "${HOME}/mygems/bin/pinecone"
+
+  assert [ ! -e "${RBENV_ROOT}/shims/lolcat" ]
+  assert [ ! -e "${RBENV_ROOT}/shims/pinecone" ]
+
+  GEM_HOME="${HOME}/mygems" run rbenv-rehash
+  assert_success ""
+
+  run ls "${RBENV_ROOT}/shims"
+  assert_success
+  assert_output <<OUT
+lolcat
+pinecone
+OUT
+}
+
 @test "carries original IFS within hooks" {
   create_hook rehash hello.bash <<SH
 hellos=(\$(printf "hello\\tugly world\\nagain"))