revlog: explicitely pass the "indexfile" parameter
Most of this was already done when introducing the `target` parameter, but some
remained. Having "indexfile" passed explicitely will help us to change the way
we address a revlog later in the stack. With the introduction of more generic
`docket`, the entry point will not necessarly be `xxx.i` file, and the actual
index files will have a variable name.
Differential Revision: https://phab.mercurial-scm.org/D10561
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