Implement git log history simplification

[mr-vinn]

When native git log is called with a pathspec, it only includes commits that are not TREESAME to any parent (TREESAME means that pathspec is not modified between the commit and its parent). If more than one parent is TREESAME to the commit, native git only follows the first one. Repository#log now implements these history simplification rules.

[coveralls]

Coverage decreased (-0.14%) when pulling f0d61a9 on mr-vinn:log-correctness into cba8de9 on gitlabhq:master.

[dosire]

/cc @randx

[dzaporozhets]

@mr-vinn does it affects performance in any way?

[mr-vinn]

It's about 40% faster on the simple benchmark from #49, using the GitLab CE repo. Probably because it doesn't examine as many commits thanks to the rule about only following one treesame parent.

Old

Repository#log with options: {:limit=>0, :path=>"public", :follow=>false}

       user     system      total        real
   0.790000   0.190000   0.980000 (  0.985883)

Found 69 commits

New

Repository#log with options: {:limit=>0, :path=>"public", :follow=>false}

       user     system      total        real
   0.460000   0.130000   0.590000 (  0.589126)

Found 28 commits

[dzaporozhets]

ok cool!