Move setup_diff_pager to libgit.a

This is used by diff-no-index.c, part of libgit.a while it stays in
builtin/diff.c. Move it to diff.c so that we won't get undefined
reference if a program that uses libgit.a happens to pull it in.

While at it, move check_pager from git.c to pager.c. It makes more
sense there and pager.c is also part of libgit.a

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Jeff King <peff@peff.net>

diff.c

@@ -4878,3 +4878,19 @@ size_t fill_textconv(struct userdiff_driver *driver,
 
 	return size;
 }
+
+void setup_diff_pager(struct diff_options *opt)
+{
+	/*
+	 * If the user asked for our exit code, then either they want --quiet
+	 * or --exit-code. We should definitely not bother with a pager in the
+	 * former case, as we will generate no output. Since we still properly
+	 * report our exit code even when a pager is run, we _could_ run a
+	 * pager with --exit-code. But since we have not done so historically,
+	 * and because it is easy to find people oneline advising "git diff
+	 * --exit-code" in hooks and other scripts, we do not do so.
+	 */
+	if (!DIFF_OPT_TST(opt, EXIT_WITH_STATUS) &&
+	    check_pager_config("diff") != 0)
+		setup_pager();
+}