Git command to list SHA1 hashes for all tree objects -

using command git show-ref --tags can see tags , sha1 hashes these tags.

i similar command trees: command output sha1 hashes tree objects, nothing else.

you can find of objects accessible head pointer

git ls-tree -r -t head 

so can filter find tree objects using sed or awk, example,

git ls-tree -r -t head | awk '$2 == "tree" { print $0 }'