test: extract some generic data and utility from test-rust-ancestor.py...
test: extract some generic data and utility from test-rust-ancestor.py
We will reuse this for more tests related to revlog index. In pratice this
series of changesets add an index implementation provided from Rust and we want
to be able to test it.
Differential Revision:
https://phab.mercurial-scm.org/D7652
