Detect git bisect in progress and display it in the prompt

[horeah]

Display suggestion: master~18/4, where the /4 could be displayed in blue or cyan and means "4 steps remaining". The easiest way to figure out the number of steps seems to be: count the lines of git bisect --visualize --oneline, then take the log2 and subtract 1 (so that we display the number of steps after the current one, just like git status displays)