t6031: generalize for recursive and resolve strategies

t6031: generalize for recursive and resolve strategies

This script tests the filemode handling of merge-recursive,
but we do not test the same thing for merge-resolve. Let's
generalize the script a little:

  1. Break out the setup steps for each test into a separate
     snippet.

  2. For each test, run it twice; once with "-s recursive"
     and once with "-s resolve". We can avoid repeating
     ourselves by adding a function.

  3. Since we have a nice abstracted function, we can make
     our tests more thorough by testing both directions
     (change on "ours" versus "theirs").

This improves our test coverage, and will make this the
place to add more tests related to merging mode changes.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>