Fix existence test in "reconcile.php"

Summary:
"rev-parse --verify" is actually a terrible test, it only survived my test cases
because I put garbage into the tables with RAND() or similar, not
properly-formatted garbage.

Use "cat-file -t" instead to ensure we perform an object-existence test.

Test Plan: Ran "reconcile.php P" locally, ran "cat-file -t" and "rev-parse
--verify" on properly-formatted but invalid hashes like
"aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa", worked with @rguerin to resolve
commit issues.

Reviewers: btrahan, rguerin

Reviewed By: rguerin

CC: aran, epriestley

Differential Revision: https://secure.phabricator.com/D1590