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".

libexec/rbenv-rehash

@@ -104,6 +104,15 @@ list_executable_names() {
       echo "${file##*/}"
     done
   done
+  # list user-install'ed executables
+  for file in ~/.gem/ruby/*/bin/*; do
+    echo "${file##*/}"
+  done
+  if [ -n "$GEM_HOME" ]; then
+    for file in "$GEM_HOME"/bin/*; do
+      echo "${file##*/}"
+    done
+  fi
 }
 
 # The basename of each argument passed to `make_shims` will be

libexec/rbenv-which

@@ -41,6 +41,17 @@ if [ "$RBENV_VERSION" = "system" ]; then
     RBENV_COMMAND_PATH="$(command -v "$RBENV_COMMAND" || true)"
 else
   RBENV_COMMAND_PATH="${RBENV_ROOT}/versions/${RBENV_VERSION}/bin/${RBENV_COMMAND}"
+  if [ ! -x "$RBENV_COMMAND_PATH" ]; then
+    # discover user-install'ed gem executables
+    if [ -n "$GEM_HOME" ]; then
+      user_install_path="${GEM_HOME}/bin/${RBENV_COMMAND}"
+    else
+      # FIXME: guessing this path is very fragile and works only for C Ruby
+      user_install_path="${HOME}/.gem/ruby/${RBENV_VERSION%.*}.0/bin/${RBENV_COMMAND}"
+    fi
+    [ -x "$user_install_path" ] && RBENV_COMMAND_PATH="$user_install_path"
+    unset user_install_path
+  fi
 fi
 
 OLDIFS="$IFS"