Fix error params in process.lua

[davidmh]

Instead of concatenating it's passing the inspected object as a second parameter.

That's throwing an error when files are staged or branches switched from outside Neogit.

[CKolkey]

good catch, though I can't recreate it by staging or switching outside neovim. Still, the second argument should be the level, not another string.

[davidmh]

Fwiw, I've seen it happen in three scenarios:

• Staging the file using vim-fugitive's :GWrite
• Cherry-picking from within a neovim terminal
• Switching into remote branches