Add support for --scan-tree=<path> option to cgit

This option makes cgit scan a directory tree looking for git repositories,
generating suitable definitions for a cgitrc file on stdout.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>

Makefile

@@ -54,6 +54,7 @@ OBJECTS += cmd.o
 OBJECTS += configfile.o
 OBJECTS += html.o
 OBJECTS += parsing.o
+OBJECTS += scan-tree.o
 OBJECTS += shared.o
 OBJECTS += ui-atom.o
 OBJECTS += ui-blob.o