Implement plain view

This implements a way to access plain blobs by path (similar to the
tree view) instead of by sha1.

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

Makefile

@@ -61,6 +61,7 @@ OBJECTS += ui-commit.o
 OBJECTS += ui-diff.o
 OBJECTS += ui-log.o
 OBJECTS += ui-patch.o
+OBJECTS += ui-plain.o
 OBJECTS += ui-refs.o
 OBJECTS += ui-repolist.o
 OBJECTS += ui-shared.o