Use abs path in gen2 butler conversion constructor

Some other parts of repo conversion change the working
directory to the gen3 butler so without this the user is
required to use an absolute path to specify their gen2
repo location.

python/lsst/obs/base/gen2to3/repoConverter.py

@@ -26,6 +26,7 @@
 from collections import defaultdict
 from abc import ABC, abstractmethod
 import fnmatch
+import os.path
 import re
 from typing import (
     Dict,
@@ -184,7 +185,8 @@ class RepoConverter(ABC):
     task : `ConvertRepoTask`
         Task instance that is using this helper object.
     root : `str`
-        Root of the Gen2 repo being converted.
+        Root of the Gen2 repo being converted.  Will be converted to an
+        absolute path, resolving symbolic links and ``~``, if necessary.
     collections : `list` of `str`
         Gen3 collections with which all converted datasets should be
         associated.
@@ -205,7 +207,7 @@ class RepoConverter(ABC):
     def __init__(self, *, task: ConvertRepoTask, root: str, run: Optional[str],
                  subset: Optional[ConversionSubset] = None):
         self.task = task
-        self.root = root
+        self.root = os.path.realpath(os.path.expanduser(root))
         self.subset = subset
         self._run = run
         self._repoWalker = None  # Created in prep