`git remote set-branches (without --add) <name> <branch>…​` does not add refspecs to `remote.<remote>.fetch` when `remote.<remote>.fetch` does not exist

Thank you for filling out a Git bug report! Please answer the following questions to help us understand your issue.

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

cd '/'; cd '/'; rm --force --recursive -- './test_git'; mkdir "$_"; cd "$_"; mkdir --parents -- './server' './client'; git -C './client' init './repo' git -C './client/repo' remote --verbose git -C './client/repo' config list --local --show-scope --show-origin git -C './client/repo' remote add server 'file://'"$(realpath './server/repo.git')" git -C './client/repo' remote --verbose git -C './client/repo' config list --local --show-scope --show-origin git -C './client/repo' remote set-branches server branch1 branch2 git -C './client/repo' config list --local --show-scope --show-origin git -C './client/repo' remote set-branches --add server branch3 branch4 git -C './client/repo' config list --local --show-scope --show-origin git -C './client/repo' remote set-branches --add server branch3 branch4 git -C './client/repo' config list --local --show-scope --show-origin git -C './client/repo' remote set-branches server git -C './client/repo' config list --local --show-scope --show-origin git -C './client/repo' remote set-branches server branch1 branch2 git -C './client/repo' config list --local --show-scope --show-origin git -C './client/repo' remote set-branches --add server branch3 branch4 git -C './client/repo' config list --local --show-scope --show-origin git -C './client/repo' remote set-branches --add server branch3 branch4 git -C './client/repo' config list --local --show-scope --show-origin

What did you expect to happen? (Expected behavior)

(local config has branch1 and branch2:) remote.server.fetch=+refs/heads/branch1:refs/remotes/server/branch1 remote.server.fetch=+refs/heads/branch2:refs/remotes/server/branch2 remote.server.fetch=+refs/heads/branch3:refs/remotes/server/branch3 remote.server.fetch=+refs/heads/branch4:refs/remotes/server/branch4 remote.server.fetch=+refs/heads/branch3:refs/remotes/server/branch3 remote.server.fetch=+refs/heads/branch4:refs/remotes/server/branch4

What happened instead? (Actual behavior)

(local config does not have branch1 and branch2:) remote.server.fetch=+refs/heads/branch3:refs/remotes/server/branch3 remote.server.fetch=+refs/heads/branch4:refs/remotes/server/branch4 remote.server.fetch=+refs/heads/branch3:refs/remotes/server/branch3 remote.server.fetch=+refs/heads/branch4:refs/remotes/server/branch4

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

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 2.46.0.windows.1
cpu: x86_64
built from commit: 2e6a859ffc0471f60f79c1256f766042b0d5d17d
sizeof-long: 4
sizeof-size_t: 8
shell-path: D:/git-sdk-64-build-installers/usr/bin/sh
feature: fsmonitor--daemon
libcurl: 8.9.0
OpenSSL: OpenSSL 3.2.2 4 Jun 2024
zlib: 1.3.1
uname: Windows 10.0 22631
compiler info: gnuc: 14.1
libc info: no libc information available
$SHELL (typically, interactive shell): C:\Program Files\Git\usr\bin\bash.exe

[Enabled Hooks] not run from a git repository - no hooks to show