libgit2 v0.22.1 Maintenance Release

The following (critical) fixes have been backported to this maintenance release.
All users of the library are encouraged to update.

• bd0e881 checkout: introduce git_checkout_perfdata
• 1ca7fa9 git_path_join_unrooted: return base len
• 9f1c873 checkout: don't recreate previous directory
• 99367df checkout tests: nasty symlinks
• b295294 checkout: drop newline in error message
• 8d65523 checkout: remove files before writing new ones
• f9df7b6 checkout tests: emulate p_realpath poorly on Win32
• 1ce9ea3 checkout tests: cleanup realpath impl on Win32