Add support for ETag in 'plain' view

When downloading a blob identified by its path, the client might want
to know if the blob has been modified since a previous download of the
same path. To this end, an ETag containing the blob SHA1 seems to be
ideal.

Todo: add support for HEAD requests...

Suggested-by: Owen Taylor <otaylor@redhat.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-shared.c

@@ -468,6 +468,8 @@ void cgit_print_http_headers(struct cgit_context *ctx)
 		      ctx->page.filename);
 	htmlf("Last-Modified: %s\n", http_date(ctx->page.modified));
 	htmlf("Expires: %s\n", http_date(ctx->page.expires));
+	if (ctx->page.etag)
+		htmlf("ETag: \"%s\"\n", ctx->page.etag);
 	html("\n");
 }