Merge branch 'grep.zsh-improvements'

Closes #5085
Closes #7451
Closes #7265
Fixes #8444
Closes #8445

lib/grep.zsh

-# is x grep argument available?
-grep-flag-available() {
-    echo | grep $1 "" >/dev/null 2>&1
-}
+__GREP_CACHE_FILE="$ZSH_CACHE_DIR"/grep-alias
 
-GREP_OPTIONS=""
+# See if there's a cache file modified in the last day
+__GREP_ALIAS_CACHES=("$__GREP_CACHE_FILE"(Nm-1))
+if [[ -z "$__GREP_ALIAS_CACHES" ]]; then
+    grep-flags-available() {
+        command grep "$@" "" &>/dev/null <<< ""
+    }
 
-# color grep results
-if grep-flag-available --color=auto; then
-    GREP_OPTIONS+=" --color=auto"
-fi
+    # Ignore these folders (if the necessary grep flags are available)
+    EXC_FOLDERS="{.bzr,CVS,.git,.hg,.svn,.idea,.tox}"
 
-# ignore VCS folders (if the necessary grep flags are available)
-VCS_FOLDERS="{.bzr,CVS,.git,.hg,.svn}"
+    # Check for --exclude-dir, otherwise check for --exclude. If --exclude
+    # isn't available, --color won't be either (they were released at the same
+    # time (v2.5): http://git.savannah.gnu.org/cgit/grep.git/tree/NEWS?id=1236f007
+    if grep-flags-available --color=auto --exclude-dir=.cvs; then
+        GREP_OPTIONS="--color=auto --exclude-dir=$EXC_FOLDERS"
+    elif grep-flags-available --color=auto --exclude=.cvs; then
+        GREP_OPTIONS="--color=auto --exclude=$EXC_FOLDERS"
+    fi
 
-if grep-flag-available --exclude-dir=.cvs; then
-    GREP_OPTIONS+=" --exclude-dir=$VCS_FOLDERS"
-elif grep-flag-available --exclude=.cvs; then
-    GREP_OPTIONS+=" --exclude=$VCS_FOLDERS"
-fi
+    {
+        if [[ -n "$GREP_OPTIONS" ]]; then
+            # export grep, egrep and fgrep settings
+            echo "alias grep='grep $GREP_OPTIONS'"
+            echo "alias egrep='egrep $GREP_OPTIONS'"
+            echo "alias fgrep='fgrep $GREP_OPTIONS'"
+        fi
+    } > "$__GREP_CACHE_FILE"
 
-# export grep settings
-alias grep="grep $GREP_OPTIONS"
+    # Clean up
+    unset GREP_OPTIONS EXC_FOLDERS
+    unfunction grep-flags-available
+fi
 
-# clean up
-unset GREP_OPTIONS
-unset VCS_FOLDERS
-unfunction grep-flag-available
+source "$__GREP_CACHE_FILE"
+unset __GREP_CACHE_FILE __GREP_ALIAS_CACHES