Merge branch 'en/filter-branch-deprecation'

Start discouraging the use of "git filter-branch".

* en/filter-branch-deprecation:
t9902: use a non-deprecated command for testing
Recommend git-filter-repo instead of git-filter-branch
t6006: simplify, fix, and optimize empty message test

diff --cc Documentation/git-rebase.txt
Simple merge

diff --cc Documentation/githooks.txt
Simple merge

diff --cc t/t9902-completion.sh
Simple merge