make git_prompt_status more accurate

[foobacca]

The git_prompt_status function only matched some of the possible output
from git status --porcelain --branch - this commit should match pretty
much the whole range, defined by the git status man page as:

X          Y     Meaning
-------------------------------------------------
          [MD]   not updated
M        [ MD]   updated in index
A        [ MD]   added to index
D         [ M]   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
!           !    ignored
-------------------------------------------------

Note that with the current variables it doesn't really distinguish
between changes in the staging area and changes in the working directory

• to do that would require a lot more ZSH_THEME_GIT_PROMPT_XYZ variables.

Also git status no longer seems to use the text "diverged" - instead it
says both "ahead" and "behind" if you have diverged. The second commit
fixes that logic.