Auto merge of #6132 - Xanewok:bump-libgit2-sys, r=Eh2406

Bump libgit2-sys to 0.7.9

For some reason I couldn't compile with newest nightly `rustc 1.31.0-nightly (5597ee8a6 2018-10-03)`:
```
error: failed to run custom build command for `libgit2-sys v0.7.7`
process didn't exit successfully: `/home/xanewok/repos/cargo/target/debug/build/libgit2-sys-f38ab3eb27549370/build-script-build` (exit code: 101)
--- stdout
running: "cmake" "/home/xanewok/.cargo/registry/src/github.com-1ecc6299db9ec823/libgit2-sys-0.7.7/libgit2" "-DGIT_SSH_MEMORY_CREDENTIALS=1" "-DBUILD_SHARED_LIBS=OFF" "-DBUILD_CLAR=OFF" "-DCMAKE_INSTALL_PREFIX=/home/xanewok/repos/cargo/target/debug/build/libgit2-sys-e96505b09ca81ecd/out" "-DCMAKE_C_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_C_COMPILER=/usr/bin/cc" "-DCMAKE_CXX_FLAGS= -ffunction-sections -fdata-sections -fPIC -m64" "-DCMAKE_CXX_COMPILER=/usr/bin/c++" "-DCMAKE_BUILD_TYPE=Debug"
-- The C compiler identification is GNU 7.3.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Found PkgConfig: /usr/bin/pkg-config (found version "0.29.1")
-- Looking for pthread.h
-- Looking for pthread.h - found
-- Looking for pthread_create
-- Looking for pthread_create - not found
-- Looking for pthread_create in pthreads
-- Looking for pthread_create in pthreads - not found
-- Looking for pthread_create in pthread
-- Looking for pthread_create in pthread - found
-- Found Threads: TRUE
-- Performing Test HAVE_STRUCT_STAT_ST_MTIM
-- Performing Test HAVE_STRUCT_STAT_ST_MTIM - Success
-- Performing Test HAVE_STRUCT_STAT_ST_MTIMESPEC
-- Performing Test HAVE_STRUCT_STAT_ST_MTIMESPEC - Failed
-- Performing Test HAVE_STRUCT_STAT_MTIME_NSEC
-- Performing Test HAVE_STRUCT_STAT_MTIME_NSEC - Failed
-- Performing Test HAVE_STRUCT_STAT_NSEC
-- Performing Test HAVE_STRUCT_STAT_NSEC - Success
-- Performing Test IS_WALL_SUPPORTED
-- Performing Test IS_WALL_SUPPORTED - Success
-- Performing Test IS_WEXTRA_SUPPORTED
-- Performing Test IS_WEXTRA_SUPPORTED - Success
-- Performing Test IS_WDOCUMENTATION_SUPPORTED
-- Performing Test IS_WDOCUMENTATION_SUPPORTED - Failed
-- Performing Test IS_WNO-MISSING-FIELD-INITIALIZERS_SUPPORTED
-- Performing Test IS_WNO-MISSING-FIELD-INITIALIZERS_SUPPORTED - Success
-- Performing Test IS_WSTRICT-ALIASING_SUPPORTED
-- Performing Test IS_WSTRICT-ALIASING_SUPPORTED - Success
-- Performing Test IS_WSTRICT-PROTOTYPES_SUPPORTED
-- Performing Test IS_WSTRICT-PROTOTYPES_SUPPORTED - Success
-- Performing Test IS_WDECLARATION-AFTER-STATEMENT_SUPPORTED
-- Performing Test IS_WDECLARATION-AFTER-STATEMENT_SUPPORTED - Success
-- Performing Test IS_WSHIFT-COUNT-OVERFLOW_SUPPORTED
-- Performing Test IS_WSHIFT-COUNT-OVERFLOW_SUPPORTED - Success
-- Performing Test IS_WNO-UNUSED-CONST-VARIABLE_SUPPORTED
-- Performing Test IS_WNO-UNUSED-CONST-VARIABLE_SUPPORTED - Success
-- Performing Test IS_WNO-UNUSED-FUNCTION_SUPPORTED
-- Performing Test IS_WNO-UNUSED-FUNCTION_SUPPORTED - Success
-- Looking for regcomp_l
-- Looking for regcomp_l - not found
-- Looking for futimens
-- Looking for futimens - found
-- Looking for qsort_r
-- Looking for qsort_r - found
-- Looking for qsort_s
-- Looking for qsort_s - not found
-- Looking for clock_gettime in rt
-- Looking for clock_gettime in rt - found
-- Checking for module 'libcurl'
--   Found libcurl, version 7.61.1
-- Configuring incomplete, errors occurred!
See also "/home/xanewok/repos/cargo/target/debug/build/libgit2-sys-e96505b09ca81ecd/out/build/CMakeFiles/CMakeOutput.log".
See also "/home/xanewok/repos/cargo/target/debug/build/libgit2-sys-e96505b09ca81ecd/out/build/CMakeFiles/CMakeError.log".
--- stderr
fatal: not a git repository (or any of the parent directories): .git
CMake Error at cmake/Modules/FindPkgLibraries.cmake:17 (MESSAGE):
  could not resolve curl
Call Stack (most recent call first):
  src/CMakeLists.txt:120 (FIND_PKGLIBRARIES)
thread 'main' panicked at '
command did not execute successfully, got: exit code: 1
build script failed, must exit now', /home/xanewok/.cargo/registry/src/github.com-1ecc6299db9ec823/cmake-0.1.29/src/lib.rs:632:5
note: Run with `RUST_BACKTRACE=1` for a backtrace.
```
on Xubuntu 18.04.1.

Simply bumping `libgit2-sys` seemed to have fixed the problem, so... :man_shrugging:

r? @alexcrichton

Author: bors

Cargo.toml

@@ -26,7 +26,7 @@ curl = { version = "0.4.17", features = ['http2'] }
 env_logger = "0.5.11"
 failure = "0.1.2"
 filetime = "0.2"
-flate2 = "1.0"
+flate2 = "1.0.3"
 fs2 = "0.4"
 git2 = "0.7.5"
 git2-curl = "0.8.1"
@@ -39,7 +39,7 @@ jobserver = "0.1.11"
 lazycell = "1.2.0"
 libc = "0.2"
 log = "0.4"
-libgit2-sys = "0.7.5"
+libgit2-sys = "0.7.9"
 num_cpus = "1.0"
 opener = "0.3.0"
 rustfix = "0.4.2"