Fix a gitignore bug

This only came up when you generate more than one package in the same
Julia session. The mutating `append!` function was adding stuff to the
global variable that the coverage plugins were using for their
gitignores. The new `vcat` is non-mutating.

src/plugins/git.jl

@@ -62,7 +62,7 @@ end
 
 # Create the .gitignore.
 function hook(p::Git, t::Template, pkg_dir::AbstractString)
-    ignore = mapreduce(gitignore, append!, t.plugins)
+    ignore = mapreduce(gitignore, vcat, t.plugins)
     # Only ignore manifests at the repo root.
     p.manifest || "Manifest.toml" in ignore || push!(ignore, "/Manifest.toml")
     unique!(sort!(ignore))