Cache credentials on Pkg up

[omus]

Avoids asking the user for their credentials multiple times for the same domain.

[omus]

Using git diff --ignore-space-change this is more comprehensible:

diff --git a/stdlib/Pkg/src/Operations.jl b/stdlib/Pkg/src/Operations.jl
index 547cebdc75..63cf3ad6f2 100644
--- a/stdlib/Pkg/src/Operations.jl
+++ b/stdlib/Pkg/src/Operations.jl
@@ -1162,6 +1162,7 @@ end
 function up(ctx::Context, pkgs::Vector{PackageSpec})
     # resolve upgrade levels to version specs
     new_git = UUID[]
+    Base.shred!(LibGit2.CachedCredentials()) do creds
         for pkg in pkgs
             if pkg.uuid in keys(ctx.stdlibs)
                 pkg.version = VersionSpec()
@@ -1172,7 +1173,8 @@ function up(ctx::Context, pkgs::Vector{PackageSpec})
             info = manifest_info(ctx.env, pkg.uuid)
             if info !== nothing && haskey(info, "repo-url")
                 pkg.repo = Types.GitRepo(info["repo-url"], info["repo-rev"])
-            new = handle_repos_add!(ctx, [pkg]; upgrade_or_add = (level == UPLEVEL_MAJOR))
+                new = handle_repos_add!(ctx, [pkg]; credentials=creds,
+                                        upgrade_or_add = (level == UPLEVEL_MAJOR))
                 append!(new_git, new)
             else
                 if info !== nothing
@@ -1192,6 +1194,7 @@ function up(ctx::Context, pkgs::Vector{PackageSpec})
                 end
             end
         end
+    end
     # resolve & apply package versions
     resolve_versions!(ctx, pkgs)
     new_apply = apply_versions(ctx, pkgs)