[BUG] log --follow ignore files renamed in a merge commit

[BUG] log --follow ignore files renamed in a merge commit

[Thibault Delor]

Hello there,

I have raised my initial issue on Stack Overflow:
https://stackoverflow.com/questions/60927159/merging-repo-with-git-subtree-prefix-break-the-history

After reading the code, it's definitely a bug with git log.
The actual issue is that if a file has been renamed inside a commit
with 2+ parents `git log --follow` won't show it and loose track of
the file. On the other side, running `git log -m --follow ...` is
working as expected.

I would be really keen to submit a patch in the next couple of days.

I am planning to tweak the method `log_tree_diff` in log_tree.c as I
feel that's where the error coming from. I am guess I will have to
force somehow the evaluation of the diff (even if the diff isn't
displayed) so the method `try_to_follow_renames` gets a chance to be
evaluated.

I have never contributed to git source code, I'll the doc
"SubmittingPatches" but feel free to give any guidance.

Thanks
Tibo