misleading message printed when worktree add fails

[Baruch Burstein <bmburstein@gmail.com>]

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Please answer the following questions to help us understand your issue.

What did you do before the bug happened? (Steps to reproduce your issue)

run `git worktree add <path> <branch>`, where <branch> is an already
checked-out branch

What did you expect to happen? (Expected behavior)

fail to create worktree

What happened instead? (Actual behavior)

worktree was not created, but a confusing message was printed:
"Preparing worktree (checking out '<branch>')"

What's different between what you expected and what actually happened?

The printed message seems to indicate that the command was a success,
when in fact
it was not

Anything else you want to add:

Please review the rest of the bug report below.
You can delete any lines you don't wish to share.


[System Info]
git version:
git version 2.33.1.windows.1
cpu: x86_64
built from commit: 05d80adb775077d673fab685009ede4a1003ed92
sizeof-long: 4
sizeof-size_t: 8
shell-path: /bin/sh
feature: fsmonitor--daemon
uname: Windows 10.0 18363
compiler info: gnuc: 10.3
libc info: no libc information available
$SHELL (typically, interactive shell): C:\Program Files\Git\usr\bin\bash.exe