Support restat when rebuilding manifest

As a fix for #874, we started reloading the entire manifest even if the
manifest was never rebuilt due to a restat rule. But this can be slow,
so call State::Reset instead, which also fixes the original crash.

Fixes #987

Test: replicate original crash, test with this change
Test: do a build that rebuilds manifest, ensure it reloads
Test: do a build that uses restat, verify that it's faster
Change-Id: Ifeada4afa1717a3691f2e787d1135c0489864629
1 file changed