[beta] Update the libgit2-sys crate

This commit is an attempt to fix an issue [1] in the latest betas for
MinGW where it appears that `libgit2` is no longer functional and always
returns errors. The attempted fix [2] has been published as a new
version of `libgit2-sys`, so this updates the beta branch to using that
commit to get it out and see if it fixes beta.

[1]: rust-lang#54206 (comment)
[2]: alexcrichton/git2-rs@9c1604e

src/Cargo.lock

@@ -198,7 +198,7 @@ dependencies = [
  "lazy_static 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "lazycell 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.43 (registry+https://github.com/rust-lang/crates.io-index)",
- "libgit2-sys 0.7.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "libgit2-sys 0.7.9 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "miow 0.3.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "num_cpus 1.8.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -825,7 +825,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "bitflags 1.0.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.43 (registry+https://github.com/rust-lang/crates.io-index)",
- "libgit2-sys 0.7.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "libgit2-sys 0.7.9 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.4 (registry+https://github.com/rust-lang/crates.io-index)",
  "openssl-probe 0.1.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "openssl-sys 0.9.35 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -1073,7 +1073,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "libgit2-sys"
-version = "0.7.8"
+version = "0.7.9"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "cc 1.0.25 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -3222,7 +3222,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 "checksum lazy_static 1.1.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ca488b89a5657b0a2ecd45b95609b3e848cf1755da332a0da46e2b2b1cb371a7"
 "checksum lazycell 1.0.0 (registry+https://github.com/rust-lang/crates.io-index)" = "d33a48d0365c96081958cc663eef834975cb1e8d8bea3378513fc72bdbf11e50"
 "checksum libc 0.2.43 (registry+https://github.com/rust-lang/crates.io-index)" = "76e3a3ef172f1a0b9a9ff0dd1491ae5e6c948b94479a3021819ba7d860c8645d"
-"checksum libgit2-sys 0.7.8 (registry+https://github.com/rust-lang/crates.io-index)" = "44b1900be992dd5698bd3bb422921e336306d413e2860e6ba3b50e62e6219c4c"
+"checksum libgit2-sys 0.7.9 (registry+https://github.com/rust-lang/crates.io-index)" = "93f2b22fce91fb820363cf88a849a8f8fdfd8be37774b6a9dd6cbda05cf940e6"
 "checksum libssh2-sys 0.2.11 (registry+https://github.com/rust-lang/crates.io-index)" = "126a1f4078368b163bfdee65fbab072af08a1b374a5551b21e87ade27b1fbf9d"
 "checksum libz-sys 1.0.22 (registry+https://github.com/rust-lang/crates.io-index)" = "65ff614643d7635dfa2151913d95c4ee90ee1fe15d9e0980f4dcb1a7e5837c18"
 "checksum lock_api 0.1.3 (registry+https://github.com/rust-lang/crates.io-index)" = "949826a5ccf18c1b3a7c3d57692778d21768b79e46eb9dd07bfc4c2160036c54"