add bindings for git_branch_name_is_valid

[franciscod]

Hello! This is my first PR for Rust code. Feedback is greatly appreciated!

related: #714

[alexcrichton]

Thanks! Is this an API though where the initialization function needs to be called first? If so this function may need to have a call to crate::init?

[franciscod]

Hmm I'm new to libgit2 as well. But let me try to answer anyway:

By looking at the function source code, it doesn't look like it needs any initialization:
https://github.com/libgit2/libgit2/blob/508361401fbb5d87118045eaeae3356a729131aa/src/branch.c#L740. After digging down some function calls, I've not seen any mention of an initialization function call being required.
Also I don't see any function in src/branch.rs or the related src/reference.rs which calls crate::init.

I'd say there's no need to call crate::init.

[alexcrichton]

Ok!

[decathorpe]

Is this function supposed to be present in libgit2 1.1.0? Because I'm getting linker errors while building the latest libgit2-sys / git2 crates (0.12.21+1.1.0 / 0.13.21) on Fedora (building against system libgit2):

git2-0.13.21/src/branch.rs:36: undefined reference to `git_branch_name_is_valid'
          collect2: error: ld returned 1 exit status

[ehuss]

No, libgit2-sys now needs latest master. That issue is tracked in #721.