fish: functions: Make it easier to review code with difftool aliases

If no arguments are provided to the git difftool aliases, just invoke
it with the default `git difftool main...current-branch`. This is what
we will use most of the time anyway as this is exactly the command
needed when we want to review our own changes before pushing upstream
or reviewing some other merge request.

fish/.config/fish/functions/gdd.fish

@@ -1,3 +1,9 @@
 function gdd --description "Use difftastic as a difftool"
+    if count $argv >/dev/null
         git difftool --tool=difftastic $argv
+    else
+        set default (git symbolic-ref refs/remotes/origin/HEAD | awk -F'/' '{print $4}')
+        set current (git branch --show-current)
+        git difftool --tool=difftastic $default...$current
+    end
 end

fish/.config/fish/functions/gdk.fish

@@ -1,3 +1,9 @@
 function gdk
+    if count $argv >/dev/null
         git difftool --tool=kitty $argv
+    else
+        set default (git symbolic-ref refs/remotes/origin/HEAD | awk -F'/' '{print $4}')
+        set current (git branch --show-current)
+        git difftool --tool=kitty $default...$current
+    end
 end

fish/.config/fish/functions/gdt.fish

@@ -1,3 +1,9 @@
 function gdt
+    if count $argv >/dev/null
         git --no-pager difftool $argv
+    else
+        set default (git symbolic-ref refs/remotes/origin/HEAD | awk -F'/' '{print $4}')
+        set current (git branch --show-current)
+        git --no-pager difftool $default...$current
+    end
 end