The testrepo test fixture has an index file that's damaged, missing an
object.  The index previously had an entry of `src/index.c` with id
3161df8cbf3a006b4ef85be6497a0ea6bde98541, but that object was missing in
the repository.  This commit adds an object to the repository and
updates the index to use that existing blob.

Similarly, the index has an entry for `readme` with an id of
97328ac7e3bd0bcd3900cb3e7a624d71dd0df888.  This can be restored from
other test repositories.

With these fixed, now the write tree from index tests can pass since they
validate object existence.

Add a new branch to the `testrepo` repository, where the `README` file
has changed to executable.  This branch enables typechange tests between
the new `executable` branch and `master`.

Add a new branch that causes a merge conflict to `testrepo` so
that we are able to test merging in worktrees.

Use legitimate (existing) object IDs in tests so that we have the
ability to turn on strict object validation when running tests.

The .gitattributes cache should not reload .gitattributes in the
middle of checking out, only between checkout operations.  Otherwise,
we'll spend all our time stat'ing and read'ing the gitattributes.

Don't try to determine whether the system supports file modes
when putting the tree data in the index during checkout.  The tree's
mode is canonical and did not come from stat(2) in the first place.