mention new shell execution behavior in release notes

This is already in the "bells and whistles" section, but it also has a
slight chance of breakage, so let's also mention it in the "changed
behaviors" section.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/RelNotes-1.7.0.txt

@@ -38,6 +38,13 @@ Notes on behaviour change
    whitespaces is reported with zero exit status when run with
    --exit-code, and there is no "diff --git" header for such a change.
 
+ * external diff and textconv helpers are now executed using the shell.
+   This makes them consistent with other programs executed by git, and
+   allows you to pass command-line parameters to the helpers. Any helper
+   paths containing spaces or other metacharacters now need to be
+   shell-quoted.  The affected helpers are GIT_EXTERNAL_DIFF in the
+   environment, and diff.*.command and diff.*.textconv in the config
+   file.
 
 Updates since v1.6.6
 --------------------