git-jump: pick a mode automatically when invoked without arguments

When `git jump` is invoked with no positional arguments (and no
arguments after `--stdout`) it currently prints usage and exits with
status 1.

But there are two situations where we can usefully infer the most
valuable and likely mode that a user would want to use, and select it
automatically:

1. When there are unmerged paths in the index, the user likely
   wants `git jump merge`.

2. When the working tree has unstaged changes, the user likely
   wants `git jump diff`.

In this commit we teach `git jump` a new "auto" mode which detects these
cases and dispatches to the corresponding mode automatically. The user
can either explicitly spell out `git jump auto`, or just leave it at
`git jump` (because "auto" is the default).

If none of the interesting cases listed above applies, then auto mode
falls back to the existing usage-and-exit behavior.

Signed-off-by: Greg Hurrell <greg.hurrell@datadoghq.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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