Fix pager mode regressions

This fixes both a segfault when starting tig in pager mode, caused by no
view being setup before calling report(). Introduced in: "Add notice
about empty pager view" (5c4358d1b309381997a5c4780a432140fefa5bcb).
Reported by Lars Noschinski in debian bug 429448.

Also, a fix for actually reading from stdin, caused by comparison of
view->vid and view->id being reported as equal leading to no actual
update. Introduced in: "Improve management of view->ref and the title
line" (035ba11f40c0a045989ed861e72327012af022b4). Now, the title line in
pager mode will say "stdin" when reading from externally piped input.