Auto merge of #2418 - alexcrichton:update-git2, r=alexcrichton

This crate was recently updated to the next release of libgit2, and I've noticed
historically that a noop `cargo build` was slow in the git2-rs repository.
Curious to see if the new libgit2 version helped speed things up at all, I
tested it out.

Before this commit, a noop `cargo build` produced 599108 syscalls. After this
commit, a noop build produced 86925 syscalls, an 85% reduction in the number of
syscalls! Needless to say it's much faster.

Author: bors

Cargo.lock

@@ -25,12 +25,12 @@ docopt = "0.6"
 env_logger = "0.3"
 filetime = "0.1"
 flate2 = "0.2"
-git2 = "0.3"
-git2-curl = "0.3"
+git2 = "0.4"
+git2-curl = "0.4"
 glob = "0.2"
 kernel32-sys = "0.2"
 libc = "0.2"
-libgit2-sys = "0.3"
+libgit2-sys = "0.4"
 log = "0.3"
 num_cpus = "0.2"
 regex = "0.1"