Merge branch 'nd/multiple-work-trees'

"git checkout [<tree-ish>] <paths>" spent unnecessary cycles
checking if the current branch was checked out elsewhere, when we
know we are not switching the branches ourselves.

* nd/multiple-work-trees:
worktree: new place for "git prune --worktrees"
checkout: don't check worktrees when not necessary

diff --cc .gitignore
Simple merge

diff --cc Makefile
Simple merge

diff --cc builtin/checkout.c
Simple merge

diff --cc builtin/gc.c
Simple merge

diff --cc builtin/prune.c
Simple merge

diff --cc command-list.txt
Simple merge

diff --cc git.c
Simple merge