### Please describe the problem.

It is not possible to export e.g. the `synced/main` branch to an exporttree=yes special remote.

### What steps will reproduce the problem?

see transcript below, but basically:


[[!format sh """
> git annex initremote myexport type=directory exporttree=yes encryption=none directory=/path/to/dir
> git config remote.myexport.annex-tracking-branch synced/main
> git annex push myexport
# Not updating export to myexport because tracking branch name is not valid. (As configured by remote.myexport.annex-tracking-branch)
"""]]


### What version of git-annex are you using? On what operating system?

10.20240927 on NixOS

### Please provide any additional information below.

[[!format sh """
yann in 🌐 yann-desktop-nixos in …/code/git-annex-test/export-synced-branch 
🐟 ❯ git init
Leeres Git-Repository in /home/yann/code/git-annex-test/export-synced-branch/.git/ initialisiert
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main 
🐟 ❯ git annex init
init  ok
(recording state in git...)
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main 
🐟 ❯ touch bla
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main [?] 
🐟 ❯ git annex assist -m "Add file"
add bla ok
(recording state in git...)
commit  (recording state in git...)
ok
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main took 2s644ms 
🐟 ❯ git annex initremote myexport type=directory exporttree=yes encryption=none directory=../exported-synced-branch
initremote myexport 
git-annex: Directory does not exist: /home/yann/code/git-annex-test/exported-synced-branch
failed
initremote: 1 failed
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main 
❌1 🐟 ❯ mkdir ../exported-synced-branch/
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main 
🐟 ❯ git annex initremote myexport type=directory exporttree=yes encryption=none directory=../exported-synced-branch
initremote myexport ok
(recording state in git...)
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main 
🐟 ❯ git config remote.myexport.annex-tracking-branch synced/main
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main 
🐟 ❯ git annex push myexport

  Not updating export to myexport because tracking branch name is not valid. (As configured by remote.myexport.annex-tracking-branch)
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main 
🐟 ❮ git config remote.myexport.annex-tracking-branch main
yann in 🌐 yann-desktop-nixos in …/export-synced-branch on  main 
🐟 ❯ git annex push myexport
export myexport bla ok
(recording state in git...)
"""]]

### Have you had any luck using git-annex before? (Sometimes we get tired of reading bug reports all day and a lil' positive end note does wonders)

git-annex is amazing, thank you! 🤩

> [[fixed|done]] --[[Joey]]
