Merge branch 'da/git-prefix-everywhere' into next

* da/git-prefix-everywhere:
t/t7503-pre-commit-hook.sh: Add GIT_PREFIX tests
git-mergetool--lib: Make vimdiff retain the current directory
git: Remove handling for GIT_PREFIX
setup: Provide GIT_PREFIX to built-ins

diff --cc git-mergetool--lib.sh
Simple merge

diff --cc git.c
Simple merge

diff --cc setup.c
Simple merge

diff --cc t/t1020-subdirectory.sh
Simple merge