Fix restat builds with edges generating headers depended on through deps files

Fix restat builds with edges generating headers depended on through deps files
in deps mode.

`ImplicitDepLoader::LoadDepFile()` already adds a depfile edge from every file
mentioned in a depfile to the depfile's output.

`ImplicitDepLoader::LoadDepsFromLog()` didn't do this yet, so add it. Else,
if a restat rule clears a generated .h file, this wouldn't propagate to cc files
depending on that .h file through a depfile.

Fixues issue #590.