ui-diff: Add links to increase/decrease number of context lines in diffs

Add two links to the "Diffstat" line to show more/less context lines in the
generated diff.

To support this, cgit_commit_link() and cgit_diff_link() has learned to
preserve the ctx.qry.context variable.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
diff --git a/ui-shared.c b/ui-shared.c
index e991799..c99bcec 100644
--- a/ui-shared.c
+++ b/ui-shared.c
@@ -341,6 +341,13 @@
 	if ((ctx.qry.ssdiff && !toggle_ssdiff) || (!ctx.qry.ssdiff && toggle_ssdiff)) {
 		html(delim);
 		html("ss=1");
+		delim = "&amp;";
+	}
+	if (ctx.qry.context > 0 && ctx.qry.context != 3) {
+		html(delim);
+		html("context=");
+		htmlf("%d", ctx.qry.context);
+		delim = "&amp;";
 	}
 	html("'>");
 	html_txt(name);
@@ -382,6 +389,13 @@
 	if ((ctx.qry.ssdiff && !toggle_ssdiff) || (!ctx.qry.ssdiff && toggle_ssdiff)) {
 		html(delim);
 		html("ss=1");
+		delim = "&amp;";
+	}
+	if (ctx.qry.context > 0 && ctx.qry.context != 3) {
+		html(delim);
+		html("context=");
+		htmlf("%d", ctx.qry.context);
+		delim = "&amp;";
 	}
 	html("'>");
 	html_txt(name);