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>

editor.py

@@ -106,11 +106,8 @@ least one of these before using this command.""", file=sys.stderr)
         raise EditorError('editor failed with exit status %d: %s %s'
           % (rc, editor, path))
 
-      fd2 = open(path)
-      try:
+      with open(path) as fd2:
         return fd2.read()
-      finally:
-        fd2.close()
     finally:
       if fd:
         os.close(fd)