git_status: Fix non-null arithmetic exit status

[ptosi]

Move from post-increment to pre-increment in the counter implementation in order to avoid the behaviour where (( X++ )) returns 1 if X evaluates to 0, which is interpreted as a failure exit
status, in turn causing the whole script to terminate prematurely.

Note that, for X non-negative, (( ++X )) never evaluates to 0, avoiding the unintended behaviour.

Reference:

$ bash -c "help '(('"

(( ... )): (( expression ))
    Evaluate arithmetic expression.

    The EXPRESSION is evaluated according to the rules for arithmetic
    evaluation.  Equivalent to "let EXPRESSION".

    Exit Status:
    Returns 1 if EXPRESSION evaluates to 0; returns 0 otherwise.