vcs_git specify binary requirement or and/or stop using porcelain commands like show

Git changed 2 things, first adding a `%B` token to the pretty print formatting in v1.7.0.4-361 that prints out the equivalent of `%s%n%b` and second changing the output of files that have been moved from one place to another from one line saying renamed to 2 saying deleted & created. We know that git version 1.7.0.4 does not work properly and that 1.7.10 does work properly.

Long term we should probably move to using lower level non porcelain commands that are less likely to spontaneously change.