Documentation: fix diff.external example

The diff.external examples pass a flag to gnu-diff, but GNU diff
does not follow the GIT_EXTERNAL_DIFF interface.

Signed-off-by: Anders Melchiorsen <mail@cup.kalibalik.dk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -92,7 +92,7 @@ Example
 
 	# Our diff algorithm
 	[diff]
-		external = "/usr/local/bin/gnu-diff -u"
+		external = /usr/local/bin/diff-wrapper
 		renames = true
 
 	[branch "devel"]
@@ -554,9 +554,11 @@ diff.autorefreshindex::
 diff.external::
 	If this config variable is set, diff generation is not
 	performed using the internal diff machinery, but using the
-	given command.  Note: if you want to use an external diff
-	program only on a subset of your files, you might want to
-	use linkgit:gitattributes[5] instead.
+	given command.  Can be overridden with the `GIT_EXTERNAL_DIFF'
+	environment variable.  The command is called with parameters
+	as described under "git Diffs" in linkgit:git[1].  Note: if
+	you want to use an external diff program only on a subset of
+	your files, you	might want to use linkgit:gitattributes[5] instead.
 
 diff.renameLimit::
 	The number of files to consider when performing the copy/rename

Documentation/git-config.txt

@@ -231,7 +231,7 @@ Given a .git/config like this:
 
 	; Our diff algorithm
 	[diff]
-		external = "/usr/local/bin/gnu-diff -u"
+		external = /usr/local/bin/diff-wrapper
 		renames = true
 
 	; Proxy settings