tests: introduce tests for grafting through renames
These cover all currently known cases of renames being grafted,
or changes being grafted through renames.
Right now, most of these cases are broken. Later patches in this series
will make them behave correctly.
The testcases heavily rely on each other, which would make it very difficult
to separate them and add them one-by-one for each case fixed by a patch.
Separating them should perhaps be a 4.1 task, if it doesn't slow down
the tests too much.
(Developed together with Pierre-Yves David)
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