Replace more git_buf internals

[QuLogic]

These functions are basically string functions that are pretty long because they handle the general case. But since the calls use a lot of hard-coded strings, it's not necessary to copy the full thing, and the result is not too much longer.

I would have like to have used snprintf for extra safety (though there really shouldn't be any issue), but it's C99 and I'm not sure if you allow that.

[coveralls]

Coverage decreased (-0.3%) to 78.101% when pulling 51e2a51 on QuLogic:less-git_buf-internals into 0eaa7fb on ropensci:master.

[QuLogic]

This leaves only the GIT_UNUSED macro and git__getenv internal functions, but the latter is a bit more complicated than these to be able to replace directly.

[stewid]

Great. I agree that snprintf is better and it shouldn't be an issue, so please go ahead and use that.
Can we add

#define GIT2R_UNUSED(x) ((void)(x))

and replace GIT_UNUSED?

Maybe there is something similar to git_getenv in the R C api, but I don't think so?

[QuLogic]

Updated to use snprintf.

[stewid]

Thanks

[stewid]

I removed the use of the GIT_UNUSED macro