`magit-interactive-resolve`: use `magit-git-lines'

Since commit f8d304a1, `magit-git-string` *actually* returns only the
first line of `git(1)`s output, causing `magit-interactive-resolve` to
fail because it now only gets the first of the three mode/SHA1 pairs it
needs to populate the `{base,our,their}-buffer's.

Instead of re-introducing something like `magit-git-output', which should
also sanitize the output (e.g. trailing newlines), use `magit-git-lines'.

Fixes #916.

Signed-off-by: Pieter Praet <pieter@praet.org>