Don't ever try to overwrite /usr/bin/git

Previously, version.sh got the path to git as its first and only argument. Since
the previous commit, this is not the path to src/version.cpp and the second
argument is the path to git.

However, let's assume someone does "git pull && make". They still have the old
Makefile laying around, which will pass the path to git as the first argument to
version.sh. Thus, this will overwrite /usr/bin/git with some generated source
code. Whoops.

Thanks to KindOne for reporting this problem!

Signed-off-by: Uli Schlachter <psychon@znc.in>