RFC Revert/delay performance regression in 'git checkout -b'

As we were integrating Git 2.23.0 into VFS for Git, we discovered that "git checkout -b new-branch" went from 0.3s to 10+s on the Windows OS repo. This was an intentional change when writing the "git switch" builtin. Here is the commit message for 65f099b ("switch: no worktree status unless real branch switch happens" 2019-03-29):

When we switch from one branch to another, it makes sense to show a summary of local changes since there could be conflicts, or some files left modified.... When switch is used solely for creating a new branch (and "switch" to the same commit) or detaching, we don't really need to show anything.

"git checkout" does it anyway for historical reasons. But we can start with a clean slate with switch and don't have to.

This essentially reverts fa655d8411 (checkout: optimize "git checkout -b <new_branch>" - 2018-08-16) and make it default for switch, but also for -B and --detach. Users of big repos are encouraged to move to switch.

I was considering doing a full, long-term revert of this change to get the performance back to normal, but I also saw this feedback on the list for this patch:

I like this last bit. The skip_merge_working_tree() function which this removes was ugly, difficult to maintain, and difficult to get just right (and easy to break -- even by changing parts of the system which one might not expect to impact it).

So, the goal is to reduce the complication given by skip_merge_working_tree() by recommending that users use 'git switch -c'. The only problem is: users will take a while to move, unless prompted.

This series does the following:

1. Reverts the change that makes 'git checkout -b' slow again.
2. Creates a warning that recommends users start using 'git switch -c' instead.

This allows us to strip out this performance feature after users have had time to adopt the new way of doing things.

Derrick Stolee (2):
Revert "switch: no worktree status unless real branch switch happens"
DEPRECATION: warn about 'git checkout -b'

 Documentation/config/checkout.txt |   8 ++
 builtin/checkout.c                | 130 +++++++++++++++++++++++++++++-
 t/t1090-sparse-checkout-scope.sh  |  14 ++++
 3 files changed, 150 insertions(+), 2 deletions(-)

base-commit: 5fa0f5238b0cd46cfe7f6fa76c3f526ea98148d9
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-317%2Fderrickstolee%2Frevert-switch-slow-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-317/derrickstolee/revert-switch-slow-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/317

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