git.c: two fixes, gitsetenv type and off-by-one error.

gitsetenv as implemented in compat/setenv.c takes two const char*
and int; match that.

Also fix an incorrect attempt in prepend_to_path() to
NUL-terminate the string which stuffed the NUL character at one
past the end of allocation, and was not needed to begin with (we
copy the old_path string including the NUL which terminates it).

Signed-off-by: Junio C Hamano <junkio@cox.net>