mirror-nixos-branch: fix git fatal: bad object error

When I wrote 30efa0a, I had the
assumption that the GIT_DIR in /home/hydra-mirror/nixpkgs-channels is
always up-to-date, but it isn't.

The actual local GIT_DIR only gets updated far further down (line 346).

This commit fixes the issue of git not yet knowing the $rev by fetching
only that $rev early, instead of moving the much more expensive git
remote update that would update all branches of nixpkgs, all the time.

Author: emilylange

mirror-nixos-branch.pl

@@ -108,6 +108,7 @@ sub fetch {
 my $rev = $evalInfo->{jobsetevalinputs}->{nixpkgs}->{revision} or die;
 
 # Get commit date of $rev as unixtime and formatted string
+run("git fetch origin $rev >&2") or die;
 my $revUnix = `git show --no-patch --format='%ct' $rev` or die;
 my $revDate = strftime("%F %T %Z", localtime($revUnix));