precise keyword matching for pretty switch

hi,

i noticed today, that the pretty-switch in git-log -diff and many other tools triggers to "oneline" if you just type an "o" after --pretty=

so, not only 

git log --pretty=oneline

but also

git log --pretty=online git log --pretty=offline git log --pretty=omfg

and finally (just to make it short *G*)

git log --pretty=osolomio

does trigger the oneline-feature. Perhaps this is not intended to work as it works now. I would recommend, that the switch should only accept the fully matched keyword and not just a lowercase "o" as the first letter of the keywords following after --pretty=, just to prevent people getting bad habbits.

I noticed this by accident, because i made a typo (i typed "online" a few times, and then noticed that an o is sufficient), perhaps nobody else noticed before.

Sincerly Nicolas