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>