doc: avoid redundant use of cat

The update-hook-example.txt script uses this anti-pattern twice. Call grep
with the input file name directy. While at it, merge the two consecutive
grep calls.

Signed-off-by: Beat Bolli <dev+git@drbeat.li>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/howto/update-hook-example.txt

@@ -100,7 +100,7 @@ info "The user is: '$username'"
 
 if test -f "$allowed_users_file"
 then
-  rc=$(cat $allowed_users_file | grep -v '^#' | grep -v '^$' |
+  rc=$(grep -Ev '^(#|$)' $allowed_users_file |
     while read heads user_patterns
     do
       # does this rule apply to us?
@@ -138,7 +138,7 @@ info "'$groups'"
 
 if test -f "$allowed_groups_file"
 then
-  rc=$(cat $allowed_groups_file | grep -v '^#' | grep -v '^$' |
+  rc=$(grep -Ev '^(#|$)' $allowed_groups_file |
     while read heads group_patterns
     do
       # does this rule apply to us?