fatal: Not a valid object name HEAD

Hello everybody,

starting with Git 2.48.0 I see some trouble with some mirrored bare
repositories. Try this:

box ~ % git clone --mirror https://github.com/codership/galera.git
Cloning into bare repository 'galera.git'...
remote: Enumerating objects: 49768, done.
remote: Counting objects: 100% (2251/2251), done.
remote: Compressing objects: 100% (553/553), done.
remote: Total 49768 (delta 1986), reused 1716 (delta 1698), pack-reused 47517 (from 3)
Receiving objects: 100% (49768/49768), 25.20 MiB | 4.92 MiB/s, done.
Resolving deltas: 100% (37386/37386), done.
box ~ % cd galera.git 
box ~/galera.git (git)-[4.x] % git describe 
release_26.4.5-345-gd811a577
box ~/galera.git (git)-[4.x] % git remote add mariadb https://github.com/MariaDB/galera.git
box ~/galera.git (git)-[4.x] % git fetch --all
Fetching origin
Fetching mariadb
remote: Enumerating objects: 638, done.
remote: Counting objects: 100% (517/517), done.
remote: Compressing objects: 100% (125/125), done.
remote: Total 638 (delta 415), reused 454 (delta 392), pack-reused 121 (from 3)
Receiving objects: 100% (638/638), 386.30 KiB | 5.08 MiB/s, done.
Resolving deltas: 100% (440/440), completed with 105 local objects.
From https://github.com/MariaDB/galera
 * [new branch]        0.6 -> mariadb/0.6
[ snipped some more branches and tags ]
box ~/galera.git (git)-[mariadb-4.x] % git describe                                                
fatal: Not a valid object name HEAD

Guess that's not expected... Why does that happen?

Possibly fallout caused by https://github.com/git/git/commit/5f212684abb66c9604e745a2296af8c4bb99961c
-- 
main(a){char*c=/*    Schoene Gruesse                         */"B?IJj;MEH"
"CX:;",b;for(a/*    Best regards             my address:    */=0;b=c[a++];)
putchar(b-1/(/*    Chris            cc -ox -xc - && ./x    */b/42*2-3)*42);}