Performance enhancement for lib/grep

- Use $ZSH_CACHE_DIR to store the grep alias with all the right features
- Expire the cache after 24 hours
- See issue #8444

lib/grep.zsh

-# is x grep argument available?
-grep-flag-available() {
+# see if we already cached the grep alias in past day
+_grep_alias_cache=("$ZSH_CACHE_DIR"/grep_alias.zsh(Nm-24))
+if (( $#_grep_alias_cache )); then
+    source "$ZSH_CACHE_DIR"/grep_alias.zsh
+else
+    # is x grep argument available?
+    grep-flag-available() {
         echo | grep $1 "" >/dev/null 2>&1
-}
+    }
 
-GREP_OPTIONS=""
+    GREP_OPTIONS=""
 
-# color grep results
-if grep-flag-available --color=auto; then
+    # color grep results
+    if grep-flag-available --color=auto; then
         GREP_OPTIONS+=" --color=auto"
-fi
+    fi
 
-# ignore these folders (if the necessary grep flags are available)
-EXC_FOLDERS="{.bzr,CVS,.git,.hg,.svn,.idea,.tox}"
+    # ignore these folders (if the necessary grep flags are available)
+    EXC_FOLDERS="{.bzr,CVS,.git,.hg,.svn,.idea,.tox}"
 
-if grep-flag-available --exclude-dir=.cvs; then
+    if grep-flag-available --exclude-dir=.cvs; then
         GREP_OPTIONS+=" --exclude-dir=$EXC_FOLDERS"
-elif grep-flag-available --exclude=.cvs; then
+    elif grep-flag-available --exclude=.cvs; then
         GREP_OPTIONS+=" --exclude=$EXC_FOLDERS"
-fi
+    fi
 
-# export grep, egrep and fgrep settings
-alias grep="grep $GREP_OPTIONS"
-alias egrep="egrep $GREP_OPTIONS"
-alias fgrep="fgrep $GREP_OPTIONS"
+    {
+        # export grep, egrep and fgrep settings
+        echo alias grep="'grep $GREP_OPTIONS'"
+        echo alias egrep="'egrep $GREP_OPTIONS'"
+        echo alias fgrep="'fgrep $GREP_OPTIONS'"
+    } > "$ZSH_CACHE_DIR/grep_alias.zsh"
 
-# clean up
-unset GREP_OPTIONS EXC_FOLDERS
-unfunction grep-flag-available
+    source "$ZSH_CACHE_DIR/grep_alias.zsh"
+
+    # clean up
+    unset GREP_OPTIONS EXC_FOLDERS
+    unfunction grep-flag-available
+fi
+unset _grep_alias_cache