Add new git-log mode

Useful in repos with more users and especially in those with abundance
of tags.

Note to self: TODO: merge with other modes

dotfiles/gittum/main.gitconfig

@@ -5,6 +5,7 @@
 [alias]
   ap = !clear && git add --patch
   br = branch
+  blog = log --graph --oneline --pretty=tformat:'%Cred%h%Creset - %C(yellow)%s%Creset %C(green)<%an>%Creset %C(blue)@%d%Creset' --abbrev-commit
   cm = commit --verbose
   cma = commit --verbose --amend
   cmf = !sh -c \