Support relative paths in --reference

Put the correctly-expanded relative paths in objects/info/alternates.
From gitrepository-layout(5), this path should be "relative to the
object database, not to the repository".

Change-Id: I7b2027ae23cf7d367b80f5a187603c4cbacdb2de
diff --git a/subcmds/init.py b/subcmds/init.py
index eeddca0..9466b9a 100644
--- a/subcmds/init.py
+++ b/subcmds/init.py
@@ -401,7 +401,7 @@
     git_require(MIN_GIT_VERSION, fail=True)
 
     if opt.reference:
-      opt.reference = os.path.abspath(os.path.expanduser(opt.reference))
+      opt.reference = os.path.expanduser(opt.reference)
 
     # Check this here, else manifest will be tagged "not new" and init won't be
     # possible anymore without removing the .repo/manifests directory.