forgit show: add key binding to display the commit message

Ctrl-T can now be used to toggle between showing the diff and the commit
message in the fzf preview window.

Also add a border label to indicate which preview is currently shown, as
well as a border showing the key bindings.

This requires fzf >= 0.49.0.

bin/git-forgit

@@ -353,7 +353,7 @@ _forgit_show_view() {
     repo=$(git rev-parse --show-toplevel)
     cd "$repo" || return 1
     echo "$input_line" | _forgit_get_files_from_diff_line | xargs -0 \
-        "$FORGIT" exec_show "${commit}" -U"$diff_context" -- | _forgit_pager diff
+        "$FORGIT" exec_show "${commit}^{commit}" -U"$diff_context" -- | _forgit_pager diff
 }
 
 _forgit_show_enter() {
@@ -383,6 +383,15 @@ _forgit_show() {
         +m -0 --bind=\"enter:execute($FORGIT show_enter {} $escaped_commit | $FORGIT pager enter)\"
         --preview=\"$FORGIT show_view {} '$_forgit_preview_context' $escaped_commit\"
         --bind=\"alt-e:execute($FORGIT edit_diffed_file {})+refresh-preview\"
+        --bind 'ctrl-t:transform:[[ ! \$FZF_PREVIEW_LABEL =~ Diff ]] &&
+                echo \"change-preview-label( Diff )+refresh-preview\" ||
+                echo \"change-preview-label( Commit Message )+refresh-preview\"'
+        --preview '[[ \$FZF_PREVIEW_LABEL =~ Diff ]] &&
+                $FORGIT show_view {} '$_forgit_preview_context' $escaped_commit ||
+                git show --quiet --color=always \${FZF_PROMPT%% *}'
+        --border=bottom
+        --border-label=' [CTRL+T] toggle preview - [ALT+E] edit file '
+        --preview-label=' Diff '
         $FORGIT_DIFF_FZF_OPTS
         --prompt=\"${commit} > \"
     "