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>

subcmds/manifest.py

@@ -40,10 +40,9 @@ in a Git repository for use during future 'repo init' invocations.
     helptext = self._helpDescription + '\n'
     r = os.path.dirname(__file__)
     r = os.path.dirname(r)
-    fd = open(os.path.join(r, 'docs', 'manifest-format.md'))
-    for line in fd:
-      helptext += line
-    fd.close()
+    with open(os.path.join(r, 'docs', 'manifest-format.md')) as fd:
+      for line in fd:
+        helptext += line
     return helptext
 
   def _Options(self, p):