Avoid AbstractString in GitError struct to make extraction inferred. (J…

…uliaLang#40402)

This significantly reduces the number of invalidated methods when
loading ProgressLogging.

stdlib/LibGit2/src/error.jl

@@ -77,7 +77,7 @@ end
 struct GitError <: Exception
  class::Class
  code::Code
- msg::AbstractString
+ msg::String
 end
 Base.show(io::IO, err::GitError) = print(io, "GitError(Code:$(err.code), Class:$(err.class), $(err.msg))")