git: new `git.ls_remote` with similar interface than other Git functions

Inspired from salt.states.git.__ls_remote__, which it aims to replace.
git: use the new git.ls_remote function

The interface of this function is similar to the other Salt Git's functions
and it uses the 'identity' settings which is set in the state.

Fixes #8094
Merge pull request #8246 from multani/fix/8094-ls-remote-use-ssh-key

State git.latest's "ls-remote" step uses the specified SSH key