Add diff-option --ext-diff

To prevent funky games with external diff engines, git-log and
friends prevent external diff engines from being called. That makes
sense in the context of git-format-patch or git-rebase.

However, for "git log -p" it is not so nice to get the message
that binary files cannot be compared, while "git diff" has no
problems with them, if you provided an external diff driver.

With this patch, "git log --ext-diff -p" will do what you expect,
and the option "--no-ext-diff" can be used to override that
setting.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.txt

@@ -168,5 +168,13 @@
 --quiet::
 	Disable all output of the program. Implies --exit-code.
 
+--ext-diff::
+	Allow an external diff helper to be executed. If you set an
+	external diff driver with gitlink:gitattributes(5), you need
+	to use this option with gitlink:git-log(1) and friends.
+
+--no-ext-diff::
+	Disallow external diff drivers.
+
 For more detailed explanation on these common options, see also
 link:diffcore.html[diffcore documentation].

diff.c

@@ -2241,6 +2241,10 @@ int diff_opt_parse(struct diff_options *options, const char **av, int ac)
 		options->exit_with_status = 1;
 	else if (!strcmp(arg, "--quiet"))
 		options->quiet = 1;
+	else if (!strcmp(arg, "--ext-diff"))
+		options->allow_external = 1;
+	else if (!strcmp(arg, "--no-ext-diff"))
+		options->allow_external = 0;
 	else
 		return 0;
 	return 1;