The macro `git_parse_error` is implemented in a variadic way so
that it's possible to pass printf-style parameters.
Unfortunately, variadic macros are not defined by C89 and thus we
cannot use that functionality. But as we have implemented
`git_error_vset` in the previous commit, we can now just use that
instead.

Convert `git_parse_error` to a variadic function and use
`git_error_vset` to fix the compliance violation. While at it,
move the function to "patch_parse.c".