last-modified: change default max-depth to 0

last-modified: change default max-depth to 0

By default git-last-modified(1) doesn't recurse into subtrees. So when
the pathspec contained a path in a subtree, the command would only print
the commit information about the parent tree of the path, like:

    $ git last-modified -- path/file
    aaa0aab1bbb2bcc3ccc4ddd5dde6eee7eff8fff9	path

Change the default behavior to give commit information about the exact
path instead:

    $ git last-modified -- path/file
    aaa0aab1bbb2bcc3ccc4ddd5dde6eee7eff8fff9	path/file

To achieve this, the default max-depth is changed to 0 and recursive is
always enabled.

The handling of option '-r' is modified to disable a max-depth,
resulting in the behavior of this option to remain unchanged.

No existing tests were modified, because there didn't exist any tests
covering the example above. But more tests are added to cover this now.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>