Add difftool aliases to gitconfig

.gitconfig

@@ -9,6 +9,7 @@
     s = status --untracked-files=no
     sa = status --untracked-files=all
     d = diff
+    gd = difftool
     dh = diff HEAD
     dc = diff --cached
     ci = commit
@@ -36,6 +37,9 @@
     changed = bold red
     untracked = white
 
+[diff]
+    tool = vimdiff
+
 [merge]
     tool = vimdiff