use open context managers in more places

Use open() as a context manager to simplify the close logic and make
the code easier to read & understand.  This is also more Pythonic.

Change-Id: I579d03cca86f99b2c6c6a1f557f6e5704e2515a7
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/244734
Reviewed-by: David Pursehouse <dpursehouse@collab.net>
Tested-by: Mike Frysinger <vapier@google.com>

git_refs.py

@@ -141,18 +141,11 @@ class GitRefs(object):
 
   def _ReadLoose1(self, path, name):
     try:
-      fd = open(path)
-    except IOError:
-      return
-
-    try:
-      try:
+      with open(path) as fd:
         mtime = os.path.getmtime(path)
         ref_id = fd.readline()
-      except (IOError, OSError):
-        return
-    finally:
-      fd.close()
+    except (IOError, OSError):
+      return
 
     try:
       ref_id = ref_id.decode()