git stat -s: short status output

git stat -s: short status output

Give -s(hort) option to "git stat" that shows the status of paths in a
more concise way.

    XY PATH1 -> PATH2

format to be more machine readable than output from "git status", which is
about previewing of "git commit" with the same arguments.

PATH1 is the path in the HEAD, and " -> PATH2" part is shown only when
PATH1 corresponds to a different path in the index/worktree.

For unmerged entries, X shows the status of stage #2 (i.e. ours) and Y
shows the status of stage #3 (i.e. theirs).  For entries that do not have
conflicts, X shows the status of the index, and Y shows the status of the
work tree.  For untracked paths, XY are "??".

    X          Y     Meaning
    -------------------------------------------------
              [MD]   not updated
    M        [ MD]   updated in index
    A        [ MD]   added to index
    D        [ MD]   deleted from index
    R        [ MD]   renamed in index
    C        [ MD]   copied in index
    [MARC]           index and work tree matches
    [ MARC]     M    work tree changed since index
    [ MARC]     D    deleted in work tree

    D           D    unmerged, both deleted
    A           U    unmerged, added by us
    U           D    unmerged, deleted by them
    U           A    unmerged, added by them
    D           U    unmerged, deleted by us
    A           A    unmerged, both added
    U           U    unmerged, both modified

    ?           ?    untracked

When given -z option, the records are terminated by NUL characters for
better machine readability.  Because the traditional long format is
designed for human consumption, NUL termination does not make sense.
For this reason, -z option implies -s (short output).

Signed-off-by: Junio C Hamano <gitster@pobox.com>