Direnv support

Direnv <https://direnv.net> is a tool for tracking your local developer
environment. With the rise of Nix popularity & its hooks for local Nix
development, this tool has become quite popular, but it caches changes
in a local directory that makes Tup noisy.

This commit is a duplicate of another proposed patch, however, unlike
that patchset, this is standalone.

src/tup/pel_group.c

@@ -53,6 +53,8 @@ int pel_ignored(const char *path, int len)
 		return 1;
 	if(len == 7 && strncmp(path, ".ccache", 7) == 0)
 		return 1;
+	if(len == 7 && strncmp(path, ".direnv", 7) == 0)
+		return 1;
 	/* See also fuse_fs.c:is_hidden() */
 	return 0;
 }

src/tup/server/fuse_fs.c

@@ -105,6 +105,8 @@ static int is_hidden(const char *path)
 		return 1;
 	if(strstr(path, "/_darcs") != NULL)
 		return 1;
+	if(strstr(path, "/.direnv") != NULL)
+		return 1;
 	if(is_ccache_path(path))
 		return 1;
 	return 0;