community/libgit2-glib: upgrade to 1.2.0

Changelog:

1.2.0
---------------
* meson: Use install_symlink
* diff: fix user_data parameter type
* ggit_repository_create_commit_from_ids: add safe checks
* Allow to sign commits
* Add macro GGIT_CHECK_VERSION
* ggit_diff_foreach: Prevent assertions with null callbacks
* Map flags related to conflicts
* meson: Fix vala symlink script
* meson: Replace calls to include_directories by strings
* meson: Use dictionaries in configuration_data objects
* meson: Use generators placeholders
* meson: Replace the package include dir path
* meson: Remove the use of join_paths function
* meson: Simplify introspection detection
* meson: Simplify enum sources generation
* doc: migrate to gi-docgen
* Add mailmap support
* Fix documentation
* Misc bug fixes