Hi,

Recent libgit has removed asize from the `struct git_buf`, which leads to this build failure:

``` gcb-plugin.c:219:12: error: no member named 'asize' in 'git_buf'; did you mean 'size'? if (buf->asize == 0) { ^~~~~ size /usr/local/include/git2/buffer.h:51:9: note: 'size' declared here size_t size; ^ gcb-plugin.c:237:10: error: no member named 'asize' in 'git_buf'; did you mean 'size'? buf->asize = 0; ^~~~~ size /usr/local/include/git2/buffer.h:51:9: note: 'size' declared here size_t size; ^ ```

Not sure how this should be fixed, since the code is explicitly using asize.

I have no knowledge of libgit API, so I'm not sure I can create a patch. I'm testing a very brutal approach, to just remove al calls to `git_buf_grow()` and it's wrapper, but I'm not sure what the consequences could be,