Handle mid in vac.

It is obviously an oversight since /sys/src/cmd/vac/file.c
was initially derived from /sys/src/cmd/fossil/file.c,
but Fossil doesn't have to copy the mid since it set
it from the uid.

It has been reviewed and merged in plan9port.

Reference: /n/sources/patch/applied/vac-file-mid
Date: Thu Dec 15 17:54:52 CET 2011
Signed-off-by: djc@9grid.fr

--- /sys/src/cmd/vac/file.c	Thu Dec 15 17:54:36 2011
+++ /sys/src/cmd/vac/file.c	Thu Dec 15 17:54:26 2011
@@ -1592,6 +1592,11 @@
 		f->dir.gid = vtstrdup(dir->gid);
 	}
 
+	if(strcmp(f->dir.mid, dir->mid) != 0){
+		vtfree(f->dir.mid);
+		f->dir.mid = vtstrdup(dir->mid);
+	}
+
 	f->dir.mtime = dir->mtime;
 	f->dir.atime = dir->atime;