Log Message:
libgit2: update to 1.0.0.

v1.0
----

This is release v1.0 "Luftschloss", which is the first stabe release of
libgit2. The API will stay compatible across all releases of the same major
version. This release includes bugfixes only and supersedes v0.99, which will
stop being maintained. Both v0.27 and v0.28 stay supported in accordance with
our release policy.

### Changes or improvements

- CMake was converted to make use of the GNUInstallDirs module for both our
  pkgconfig and install targets in favor of our custom build options
  `BIN_INSTALL_DIR`, `LIB_INSTALL_DIR` and `INCLUDE_INSTALL_DIR`. Instead, you
  can now use CMakes standard variables `CMAKE_INSTALL_BINDIR`,
  `CMAKE_INSTALL_LIBDIR` and `CMAKE_INSTALL_INCLUDEDIR`.

- Some CMake build options accepted either a specific value or a boolean value
  to disable the option altogether or use automatic detection. We only accepted
  "ON" or "OFF", but none of the other values CMake \ 
recognizes as boolean. This
  was aligned with CMake's understanding of booleans.

- The installed pkgconfig file contained incorrect values for both `libdir` and
  `includedir` variables.

- If using pcre2 for regular expressions, then we incorrectly added "pcre2"
  instead of "pcre2-8" to our pkgconfig dependencies, which was corrected.

- Fixed building the bundled ntlmclient dependency on FreeBSD, OpenBSD and
  SunOS.

- When writing symlinks on Windows, we incorrectly handled relative symlink
  targets, which was corrected.

- When using the HTTP protocol via macOS' SecureTransport implementation, reads
  could stall at the end of the session and only continue after a timeout of 60
  seconds was reached.

- The filesystem-based reference callback didn't corectly initialize the backend
  version.

- A segmentation fault was fixed when calling `git_blame_buffer()` for files
  that were modified and added to the index.

- A backwards-incompatible change was introduced when we moved some structures
  from "git2/credentials.h" into "git2/sys/credentials.h". \ 
This was fixed in the
  case where you do not use hard deprecation.

- Improved error handling in various places.