Issue 533. MkRepoCommits.compare() implemented.
Implementation
Merge branch 'master' into #589
Merge branch 'master' of github.com:aistomin/jcabi-github into Issue-553
#625 first round. added @NotNull almost everywhere
#625 second round. Added @NotNull to more methods
#625 Added rest of @NotNull

Also, put some puzzles and Ignored some tests.
Nullability test still stays ignored due to bugs.
#625 checkstyle fixes
#268 iterate() method for MkRepoCommits
#268 style fixes
Issue #636 Randomized tag name for creation and patch
#642 reproduced
#642 fixed
Merge branch 'master' into 625
Merge remote-tracking branch 'upstream/master'
Merge branch 'master' into 625
Merge remote-tracking branch 'upstream/master'
#625 merge master into 625
#625 added @NotNull to mock classes
#625 checkstyle fixes