Reliably detect parent shell in `pyenv init` (fixes #93)

Imported changes from sstephenson/rbenv#453.

libexec/pyenv-init

@@ -22,7 +22,9 @@ 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="${shell##-}"
+  shell="$(basename "${shell:-$SHELL}")"
 fi
 
 READLINK=$(type -p greadlink readlink | head -1)
@@ -75,7 +77,7 @@ if [ -z "$print" ]; then
     echo
     case "$shell" in
     fish )
-      echo '. (pyenv init -|psub)'
+      echo 'status --is-interactive; and . (pyenv init -|psub)'
       ;;
     * )
       echo 'eval "$(pyenv init -)"'
@@ -100,8 +102,22 @@ if [[ ":${PATH}:" != *:"${PYENV_ROOT}/shims":* ]]; then
   esac
 fi
 
+case "$shell" in
+fish )
+  echo "setenv PYENV_SHELL $shell"
+;;
+* )
+  echo "export PYENV_SHELL=$shell"
+;;
+esac
+
 completion="${root}/completions/pyenv.${shell}"
-[ -r "$completion" ] && echo ". '$completion'"
+if [ -r "$completion" ]; then
+  case "$shell" in
+  fish ) echo ". '$completion'" ;;
+  *    ) echo "source '$completion'" ;;
+  esac
+fi
 
 if [ -z "$no_rehash" ]; then
   echo 'pyenv rehash 2>/dev/null'

libexec/pyenv-sh-rehash

@@ -7,7 +7,7 @@ if [ "$1" = "--complete" ]; then
   exec pyenv-rehash --complete
 fi
 
-shell="$(basename "$SHELL")"
+shell="$(basename "${PYENV_SHELL:-$SHELL}")"
 
 # When pyenv shell integration is enabled, delegate to pyenv-rehash,
 # then tell the shell to empty its command lookup cache.
@@ -15,11 +15,9 @@ pyenv-rehash
 
 case "$shell" in
 fish )
-  or='; or'
+  # no rehash support
   ;;
 * )
-  or='||';
+  echo "hash -r 2>/dev/null || true";
   ;;
 esac
-
-echo "hash -r 2>/dev/null $or true"

libexec/pyenv-sh-shell

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