`git delete-merged-branches` can delete default branch such as `master`

When running git delete-merged-branches on a non default branch, it currently deletes the default branch:

$ git switch -c InlayHintsPassFactory-mayhem
$ git delete-merged-branches
Deleted branch master (was 81901fca2).

It's not a big issue, as it can be recovered by switching to master again, looking at the ref log if needed. But this might be something to fix.

It's possible to get the default branch of the remote named origin

$ git rev-parse --abbrev-ref origin/HEAD
origin/master

If the remote are not named origin this could fail

$ git remote show
bric3
upstream
$ git rev-parse --abbrev-ref origin/HEAD
origin/HEAD
fatal: ambiguous argument 'origin/HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'

And not all remote have a default branch

$ git rev-parse --abbrev-ref bric3/HEAD
bric3/HEAD
fatal: ambiguous argument 'bric3/HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions, like this:
'git <command> [<revision>...] -- [<file>...]'
$ ls .git/refs/remotes/bric3/HEAD
".git/refs/remotes/bric3/HEAD": No such file or directory (os error 2)
$ git rev-parse --abbrev-ref upstream/HEAD
upstream/master
$ ls .git/refs/remotes/upstream/HEAD
.git/refs/remotes/upstream/HEAD

Start by locating the implementation of git delete-merged-branches and reproduce the reported deletion from a non-default branch. Check the behavior with origin, non-origin remotes, and remotes without a default branch; it is done when the repository's default branch is not deleted while eligible merged branches still are.