Implement Git alias to list commits ahead/behind tracking branch

There was only the `lnew` alias that lists commit behind `ORIG_HEAD` (1)
which only works after a solved merge conflict.
This is not very useful and has been replaced by two new aliases:

- `lahead` - Lists all commits the current branch is ahead of the
  tracked origin branch that have not been pushed yet.
- `lbehind` - Lists all ahead commits of the tracked origin branch that
  are not merged into the current branch yet.

References:
  (1) https://git-scm.com/docs/gitrevisions#gitrevisions

Resolves GH-180

snowblocks/git/gitconfig

@@ -408,6 +408,22 @@
   #   <ABBREVIATED_SHA1> <COMMIT_MESSAGE> <REFS>
   l = log --graph --pretty=format:'%C(cyan)%h %C(white)%s %C(cyan bold)%d'
 
+  # Lists all commits the current branch is ahead of the tracked origin branch that have not been pushed yet.
+  #
+  # Depends on:
+  #   alias.l
+  # Usage:
+  #   git lahead
+  lahead = ! git l @{u}..HEAD
+
+  # Lists all ahead commits of the tracked origin branch that are not merged into the current branch yet.
+  #
+  # Depends on:
+  #   alias.l
+  # Usage:
+  #   git lbehind
+  lbehind = ! git l HEAD..@{u}
+
   # Prints a prettified flat commit log without topic commits from merges.
   #
   # Depends on:
@@ -443,14 +459,6 @@
   #   git lch <TAG>|<SHA1>
   lch = ! git l $1..HEAD
 
-  # Lists all new commits after the fetch including stats.
-  #
-  # Depends on:
-  #   alias.l
-  # Usage:
-  #   git lnew
-  lnew = ! git l ORIG_HEAD.. --stat
-
 # +----------------------+
 # + Information - Status +
 # +----------------------+