manifest: apply checkambig=True only for root 00manifest.i This is a fix for my 14ad8e2a4abe, which used 'bool(dir)' as checkambig value for revlog.__init__(). I can't remember why I did so in 14ad8e2a4abe, but this is obviously wrong, because only root indexfile is cached via filecache-ed property of localrepository.

revlog: address review feedback for deltachain C implementation * Scope of "value" is reduced * index_baserev() is documented * Error is no longer redundantly set for -2 return values * Error values are compared <= -2 instead of == -2 to protect against odd failure scenarios

test-rebase-interruptions: stabilize for Windows External hooks end up launching cmd.exe, which knows nothing about $VAR syntax. For some reason, I thought that Mercurial would substitute in the value, in order to paper over the platform difference. But I can't find that in the documentation, and there's at least one other use of this pattern [1]. [1] https://www.mercurial-scm.org/repo/hg/file/tip/tests/test-histedit-fold.t#l477