get_oid(): when an object was not found, try harder

It is quite possible that the loose object cache gets stale when new
objects are written. In that case, get_oid() would potentially say that
it cannot find a given object, even if it should find it.

Let's blow away the loose object cache as well as the read packs and try
again in that case.

Note: this does *not* affect the code path that was introduced to help
avoid looking for the same non-existing objects (which made some
operations really expensive via NFS): that code path is handled by the
`OBJECT_INFO_QUICK` flag (which does not even apply to `get_oid()`,
which has no equivalent flag, at least at the time this patch was
written).

This incidentally fixes the problem identified earlier where an
interactive rebase wanted to re-read (and validate) the todo list after
an `exec` command modified it.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

sha1-name.c

@@ -442,6 +442,18 @@ static enum get_oid_result get_short_oid(const char *name, int len,
 	find_short_packed_object(&ds);
 	status = finish_object_disambiguation(&ds, oid);
 
+	/*
+	 * If we didn't find it, do the usual reprepare() slow-path,
+	 * since the object may have recently been added to the repository
+	 * or migrated from loose to packed.
+	 */
+	if (status == MISSING_OBJECT) {
+		reprepare_packed_git(the_repository);
+		find_short_object_filename(&ds);
+		find_short_packed_object(&ds);
+		status = finish_object_disambiguation(&ds, oid);
+	}
+
 	if (!quietly && (status == SHORT_NAME_AMBIGUOUS)) {
 		struct oid_array collect = OID_ARRAY_INIT;
 

t/t3429-rebase-edit-todo.sh

@@ -11,7 +11,7 @@ test_expect_success 'rebase exec modifies rebase-todo' '
 	test -e F
 '
 
-test_expect_failure SHA1 'loose object cache vs re-reading todo list' '
+test_expect_success SHA1 'loose object cache vs re-reading todo list' '
 	GIT_REBASE_TODO=.git/rebase-merge/git-rebase-todo &&
 	export GIT_REBASE_TODO &&
 	write_script append-todo.sh <<-\EOS &&