LibGit2: Try collecting github sideband messages

[IanButterworth]

github sends "sideband" messages to git clone that would be helpful for Pkg to show.
See the remote: prefixed messages here.

% git clone https://github.com/IanButterworth/Private.jl
Cloning into 'Private.jl'...
Username for 'https://github.com': IanButterworth
Password for 'https://IanButterworth@github.com':
remote: Support for password authentication was removed on August 13, 2021.
remote: Please see https://docs.github.com/get-started/getting-started-with-git/about-remote-repositories#cloning-with-https-urls for information on currently recommended modes of authentication.
fatal: Authentication failed for 'https://github.com/IanButterworth/Private.jl/'

This PR tries to capture and show them as part of our GitError (trying to fix JuliaLang/Pkg.jl#2701), but currently it doesn't detect any messages. I'm not sure why not.

The error below isn't relevant if you have github auth setup, it's only about user/password entry, but I thought maybe we should generally capture things github etc. is telling us.

Note LibGit2 tries 3 times.

julia> LibGit2.clone("https://github.com/IanButterworth/Private.jl", "/tmp/test_private_clone"; callbacks=LibGit2.Callbacks())
Username for 'https://github.com': IanButterworth
Password for 'https://IanButterworth@github.com': 
Username for 'https://github.com' [IanButterworth]:
Password for 'https://IanButterworth@github.com': 
Username for 'https://github.com' [IanButterworth]: 
Password for 'https://IanButterworth@github.com': 
ERROR: GitError: Code:EAUTH, Class:Callback, Message:
Aborting, maximum number of prompts reached.
Stacktrace:
 [1] macro expansion
   @ ~/Documents/GitHub/julia/usr/share/julia/stdlib/v1.13/LibGit2/src/error.jl:124 [inlined]
 [2] clone(repo_url::String, repo_path::String, clone_opts::LibGit2.CloneOptions)
   @ LibGit2 ~/Documents/GitHub/julia/usr/share/julia/stdlib/v1.13/LibGit2/src/repository.jl:470
 [3] clone(repo_url::String, repo_path::String; branch::String, isbare::Bool, remote_cb::Ptr{Nothing}, credentials::Nothing, callbacks::Dict{Symbol, Tuple{…}})
   @ LibGit2 ~/Documents/GitHub/julia/usr/share/julia/stdlib/v1.13/LibGit2/src/LibGit2.jl:592
 [4] top-level scope
   @ REPL[2]:1
Some type information was truncated. Use `show(err)` to see complete types.

(Note Base.getpass doesn't currently add a newline after entry like Base,prompt does. I think that's a bug which is tentatively fixed here..)