Inconsistency in symlink path

Some symlink diffs show the symlink destination path as a relative path, others as an absolute path. It'd be nice to be consistent.

as per @kuychaco:

Here's a tiny inconsistency in how we present the symlink path (relative vs. absolute) -- no big deal really, just wanted to point it out. We don't have to resolve it for this PR. Could make a good first issue for someone in the community if we don't get around to it before merge.

Originally reported by @kuychaco in https://github.com/atom/github/pull/1512#issuecomment-430437777.