reliably detect parent shell in rbenv init

`$SHELL` variable is a terrible way of detecting the current shell
because it's not even supposed to reflect the current shell; it's meant
for keeping the value of the default shell for programs to start.

If an explicit `<shell>` argument wasn't passed to `rbenv init`, it
tries to detect the shell by getting the name of its parent process. If
this fails, it falls back on the value of `$SHELL` as before.

Furthermore, `rbenv init` will set the RBENV_SHELL variable in the
current shell to the value of the detected shell so that `sh-shell` and
`sh-rehash` commands don't have to repeat the detection.

libexec/rbenv-init

@@ -22,7 +22,8 @@ done
 
 shell="$1"
 if [ -z "$shell" ]; then
-  shell="$(basename "$SHELL")"
+  shell="$(ps c -p $(ps -p $$ -o 'ppid=' 2>/dev/null) -o 'comm=' 2>/dev/null || true)"
+  shell="$(basename "${shell:-$SHELL}")"
 fi
 
 READLINK=$(type -p greadlink readlink | head -1)
@@ -100,6 +101,15 @@ if [[ ":${PATH}:" != *:"${RBENV_ROOT}/shims":* ]]; then
   esac
 fi
 
+case "$shell" in
+fish )
+  echo "setenv RBENV_SHELL $shell"
+;;
+* )
+  echo "export RBENV_SHELL=$shell"
+;;
+esac
+
 completion="${root}/completions/rbenv.${shell}"
 if [ -r "$completion" ]; then
   case "$shell" in

libexec/rbenv-sh-rehash

@@ -7,7 +7,7 @@ if [ "$1" = "--complete" ]; then
   exec rbenv-rehash --complete
 fi
 
-shell="$(basename "$SHELL")"
+shell="$(basename "${RBENV_SHELL:-$SHELL}")"
 
 # When rbenv shell integration is enabled, delegate to rbenv-rehash,
 # then tell the shell to empty its command lookup cache.

libexec/rbenv-sh-shell

@@ -24,7 +24,7 @@ if [ "$1" = "--complete" ]; then
 fi
 
 version="$1"
-shell="$(basename "$SHELL")"
+shell="$(basename "${RBENV_SHELL:-$SHELL}")"
 
 if [ -z "$version" ]; then
   if [ -z "$RBENV_VERSION" ]; then