Retire support for old environment variables.

We have deprecated the old environment variable names for quite a
while and now it's time to remove them.  Gone are:

    SHA1_FILE_DIRECTORIES AUTHOR_DATE AUTHOR_EMAIL AUTHOR_NAME
    COMMIT_AUTHOR_EMAIL COMMIT_AUTHOR_NAME SHA1_FILE_DIRECTORY

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/diff.c b/diff.c
index 527cbf0..71696c5 100644
--- a/diff.c
+++ b/diff.c
@@ -32,10 +32,10 @@
 	 *
 	 * GIT_DIFF_OPTS="-c";
 	 */
-	external_diff_cmd = gitenv("GIT_EXTERNAL_DIFF");
+	external_diff_cmd = getenv("GIT_EXTERNAL_DIFF");
 
 	/* In case external diff fails... */
-	env_diff_opts = gitenv("GIT_DIFF_OPTS");
+	env_diff_opts = getenv("GIT_DIFF_OPTS");
 	if (env_diff_opts) diff_opts = env_diff_opts;
 
 	done_preparing = 1;