Add rename detection & async filling for file revlog

[naseschwarz]

This is an initial draft for an update to #1881. I've rebased onto the current master, connected the output path, fixed some first issues and made clippy happy.

There's still a lot to do, but this should already work for testing the UI.

This Pull Request fixes/closes #{issue_num}.

It changes the following:

I followed the checklist:

• I added unittests
• I ran make check without errors
• I tested the overall application
• I added an appropriate item to the changelog

[naseschwarz]

The current algorithm does not correctly deal with renames in branches:

git init
echo a > a
git add a
git commit -m '1'
git mv a b
git commit -m 'a -> b'
git checkout main~ -b funky-branch
echo some-line >> a
git add a
git commit -m '2'
git checkout -
git mv b a
git commit -m 'b -> a'
git merge --no-ff funky-branch

File revlog of gitui in master shows all five commits, with rename detection, the third commit adding "some-line" to "a" is missed.

git log --follow a finds all commits (and skips the merge commit, while gitui includes it):

@extrawurst: I would argue that until the issue missing an obvious change to a file is fixed, we either should abstain from merging this or we should make this feature optional. Rename following can at most be done heuristically anyhow and I'd suggest that it should always be optional, even if the issue above would be resolved.

It should be straightforward to resolve this with a graph-aware log walker, though.

(Also, have not checked, just a note for myself: Currently, the log walker orders commit by time. It should use topological ordering instead.)