Add git difftool alias

See #1617

aliases/available/git.aliases.bash

@@ -37,6 +37,7 @@ alias grv='git remote -v'
 alias gra='git remote add'
 alias gd='git diff'
 alias gds='git diff --staged'
+alias gdt='git difftool'
 alias gdv='git diff -w "$@" | vim -R -'
 alias gc='git commit -v'
 alias gca='git commit -v -a'