Add binding for git_repository_open_ext

[joshtriplett]

This addresses part of #123.

[joshtriplett]

I intentionally didn't include the version bumps for libgit2-sys or git2 in this pull request, because I think it makes sense to wait until libgit2/libgit2#3711 and then the rest of #123 get merged, and then bump both to 0.5.0 given the new APIs.

[alexcrichton]

I believe that this is actually a naturally-separated list of directories, so perhaps this could be an iterator of T: AsRef<OsStr>? I think there's some other APIs for consuming an iterator like that elsewhere as well.

[joshtriplett]

There's iter2cstrs, but that just constructs a git_strarray. git_repository_open_ext takes a single string of paths separated by GIT_PATH_LIST_SEPARATOR.

Also, if the caller already has this parameter in the form libgit2 expects, it seems unfortunate to have to split it just to have git2-rs re-join it.

I think it makes more sense for the caller to call std::env::join_paths themselves, rather than having open_ext do it for them; that way, if you already have a pre-joined path in the form you'd get it from $GIT_CEILING_DIRECTORIES, you can just pass that.

[alexcrichton]

That all sounds good to me, thanks @joshtriplett!

[joshtriplett]

I expanded the documentation for git::Repository::open_ext, including documentation for all the flags, documentation for ceiling_dirs, and a reference to std::env for function to construct a path list for ceiling_dirs.

[alexcrichton]

I actually tend to use the bound IntoCString in this repo nad that should help elide the .as_ref() below as well

[joshtriplett]

For the path? Sure, I can do that. I'd modeled this after other functions in git::Repository, which also took AsRef<Path>, but IntoCString seems easy enough to switch to.

[alexcrichton]

Oh weird, I should probably change those to IntoCString at some point. In that case feel free to leave this :)

[joshtriplett]

OK, will do.

[joshtriplett]

OK, the version I just pushed uses an IntoIterator, where the Item is an AsRef<OsStr>.

I didn't manage to get an IntoIterator over IntoCString to work (specifically, the conversion to the types join_paths wanted). Please feel free to generalize if you'd like.

I also find it unfortunate that, in the common case of not passing ceiling_dirs, I still had to say &[] as &[&OsStr], because &[] didn't provide enough information for type inference to work.

[alexcrichton]

Oh yeah AsRef<OsStr> is fine if we're calling join_paths. That is pretty unfortunate about calling this with an empty slice, but in theory that won't happen most of the time as this is just a "power user" function over one of the more ergonomic variants?

Gonna go ahead and merge for now, thanks @joshtriplett!

[alexcrichton]

oops