Do not use "diff" found on PATH while building and installing

Some of the flags used with the first diff found in PATH cause the
vendor diff to choke.

Signed-off-by: Gary V. Vaughan <gary@thewrittenword.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/install-webdoc.sh

@@ -12,7 +12,7 @@ do
 	then
 		: did not match
 	elif test -f "$T/$h" &&
-	   diff -u -I'Last updated [0-9][0-9]-[A-Z][a-z][a-z]-' "$T/$h" "$h"
+	   $DIFF -u -I'Last updated [0-9][0-9]-[A-Z][a-z][a-z]-' "$T/$h" "$h"
 	then
 		:; # up to date
 	else