Git fetcher: Don't update mtime of ref file if fetching by rev

This fixes the warning

  $ nix eval --store /tmp/nix --expr 'builtins.fetchTree { type = "git"; url = "https://github.com/DeterminateSystems/attic"; ref = "fixups-for-magic-nix-cache"; rev = "635753a2069d4b8228e846dc5c09ad361c75cd1a"; }'
  warning: could not update mtime for file '/home/eelco/.cache/nix/gitv3/09788h9zgba5lbfkaa6ija2dvi004jwsqjf5ln21i2njs07cz766/refs/heads/fixups-for-magic-nix-cache': error: changing modification time of '"/home/eelco/.cache/nix/gitv3/09788h9zgba5lbfkaa6ija2dvi004jwsqjf5ln21i2njs07cz766/refs/heads/fixups-for-magic-nix-cache"': No such file or directory

When we're fetching by rev, that file doesn't necessarily exist, and we
don't care about it anyway.

src/libfetchers/git.cc

@@ -584,9 +584,10 @@ struct GitInputScheme : InputScheme
                 }
 
                 try {
-                    setWriteTime(localRefFile, now, now);
+                    if (!input.getRev())
+                        setWriteTime(localRefFile, now, now);
                 } catch (Error & e) {
-                    warn("could not update mtime for file '%s': %s", localRefFile, e.msg());
+                    warn("could not update mtime for file '%s': %s", localRefFile, e.info().msg);
                 }
                 if (!originalRef && !storeCachedHead(repoInfo.url, ref))
                     warn("could not update cached head '%s' for '%s'", ref, repoInfo.url);