feat(xact): add default time when copying transaction

Use the transaction's time as default time when asking when the copy should be
created.  This allows us to use the relative syntax of org-read-date to move the
transaction an <interval> forward or backward from the time of the transaction:

+1d (+ 1 day from TODAY)
++1d (+ 1 day from DEFAULT-TIME, i.e. date of the original transaction)

In other words, single +/- is relative against today and double ++/-- is
relative against the DEFAULT-TIME.

Note that since currently the DEFAULT-TIME is today, this feature is only
strictly adding functionality and should therefore be fully backward-compatible.

Author: Fuco1

ledger-mode.el

@@ -92,11 +92,12 @@
                                            (regexp-quote account))
                                          (ledger-accounts-list))))
 
-(defun ledger-read-date (prompt)
+(defun ledger-read-date (prompt &optional default-time)
   "Return user-supplied date after `PROMPT', defaults to today.
 This uses `org-read-date', which see."
-  (ledger-format-date (let ((org-read-date-prefer-future nil))
-                        (org-read-date nil t nil prompt))))
+  (ledger-format-date (let ((org-read-date-prefer-future nil)
+                            (org-extend-today-until 0))
+                        (org-read-date nil t nil prompt default-time))))
 
 (defun ledger-get-minibuffer-prompt (prompt default)
   "Return a minibuffer prompt string composing PROMPT and DEFAULT."

ledger-xact.el

@@ -151,7 +151,9 @@ MOMENT is an encoded date"
   "Ask for a new DATE and copy the transaction under point to that date.
 Leave point on the first amount."
   (interactive  (list
-                 (ledger-read-date "Copy to date: ")))
+                 (ledger-read-date "Copy to date: "
+                                   (ledger-parse-iso-date
+                                    (ledger-xact-date)))))
   (let* ((extents (ledger-navigate-find-xact-extents (point)))
          (transaction (buffer-substring-no-properties (car extents) (cadr extents)))
          (encoded-date (ledger-parse-iso-date date)))