Previously when calling `glog -- file` with a file that was renamed
sometime in the history, the preview would just be empty for all commits
before the rename, since it's path didn't exist.
Fix this by checking for empty output and falling back to the full patch
in that case.
This also heavily refactors the code around `$fzf_preview` to make it
more readable.
TODO: It would be nice if git-show would fail in this case instead of
just printing nothing and returning zero
Julian Prein
00d28845a3
