replace: Completely merge given priority / date with existing

So that any combination of priority / date entered in the replacement will replace the corresponding original ones, but if they are left out, the original ones will be kept.
In essence, omitted stuff will be kept, added stuff will override, only deletion of existing stuff is not possible (but this is replace, after all).

Fixes #386

tests/t1100-replace.sh

@@ -140,6 +140,18 @@ TODO: Replaced task with:
 1 2010-07-04 this also has a new date
 EOF
 
+cat /dev/null > todo.txt
+test_todo_session 'replace handling prepended priority on add' <<EOF
+>>> todo.sh -t add "new task"
+1 2009-02-13 new task
+TODO: 1 added.
+
+>>> todo.sh replace 1 '(B) this also has a priority now'
+1 2009-02-13 new task
+TODO: Replaced task with:
+1 (B) 2009-02-13 this also has a priority now
+EOF
+
 cat /dev/null > todo.txt
 test_todo_session 'replace handling priority and prepended date on add' <<EOF
 >>> todo.sh -t add "new task"
@@ -156,6 +168,18 @@ TODO: Replaced task with:
 1 (A) 2009-02-13 this is just a new one
 EOF
 
+cat /dev/null > todo.txt
+test_todo_session 'replace handling prepended priority and date on add' <<EOF
+>>> todo.sh -t add "new task"
+1 2009-02-13 new task
+TODO: 1 added.
+
+>>> todo.sh replace 1 '(C) 2010-07-04 this also has a priority and new date'
+1 2009-02-13 new task
+TODO: Replaced task with:
+1 (C) 2010-07-04 this also has a priority and new date
+EOF
+
 echo '(A) 2009-02-13 this is just a new one' > todo.txt
 test_todo_session 'replace with prepended date replaces existing date' <<EOF
 >>> todo.sh replace 1 2010-07-04 this also has a new date
@@ -164,6 +188,14 @@ TODO: Replaced task with:
 1 (A) 2010-07-04 this also has a new date
 EOF
 
+echo '(A) 2009-02-13 this is just a new one' > todo.txt
+test_todo_session 'replace with prepended priority replaces existing priority' <<EOF
+>>> todo.sh replace 1 '(B) this also has a new priority'
+1 (A) 2009-02-13 this is just a new one
+TODO: Replaced task with:
+1 (B) 2009-02-13 this also has a new priority
+EOF
+
 echo '2009-02-13 this is just a new one' > todo.txt
 test_todo_session 'replace with prepended priority and date replaces existing date' <<EOF
 >>> todo.sh replace 1 '(B) 2010-07-04 this also has a new date'
@@ -172,4 +204,13 @@ TODO: Replaced task with:
 1 (B) 2010-07-04 this also has a new date
 EOF
 
+
+echo '(A) 2009-02-13 this is just a new one' > todo.txt
+test_todo_session 'replace with prepended priority and date replaces existing priority and date' <<EOF
+>>> todo.sh replace 1 '(B) 2010-07-04 this also has a new prio+date'
+1 (A) 2009-02-13 this is just a new one
+TODO: Replaced task with:
+1 (B) 2010-07-04 this also has a new prio+date
+EOF
+
 test_done

todo.sh

@@ -459,20 +459,31 @@ replaceOrPrepend()
 
   # Retrieve existing priority and prepended date
   local -r priAndDateExpr='^\((.) \)\{0,1\}\([0-9]\{2,4\}-[0-9]\{2\}-[0-9]\{2\} \)\{0,1\}'
-  priority=$(sed -e "$item!d" -e "${item}s/${priAndDateExpr}.*/\\1/" "$TODO_FILE")
-  prepdate=$(sed -e "$item!d" -e "${item}s/${priAndDateExpr}.*/\\2/" "$TODO_FILE")
-
-  if [ "$prepdate" ] && [ "$action" = "replace" ] && [ "$(echo "$input"|sed -e "s/${priAndDateExpr}.*/\\1\\2/")" ]; then
+  originalPriority=$(sed -e "$item!d" -e "${item}s/${priAndDateExpr}.*/\\1/" "$TODO_FILE")
+  priority="$originalPriority"
+  originalPrepdate=$(sed -e "$item!d" -e "${item}s/${priAndDateExpr}.*/\\2/" "$TODO_FILE")
+  prepdate="$originalPrepdate"
+  if [ "$action" = "replace" ]; then
+    replacementPrepdate="$(echo "$input"|sed -e "s/${priAndDateExpr}.*/\\2/")"
+    if [ "$replacementPrepdate" ]; then
       # If the replaced text starts with a [priority +] date, it will replace
       # the existing date, too.
-    prepdate=
+      prepdate="$replacementPrepdate"
+    fi
+    replacementPriority="$(echo "$input"|sed -e "s/${priAndDateExpr}.*/\\1/")"
+    if [ "$replacementPriority" ]; then
+      # If the replaced text starts with a priority, it will replace
+      # the existing priority, too.
+      priority="$replacementPriority"
+    fi
+    input="$(echo "$input"|sed -e "s/${priAndDateExpr}//")"
   fi
 
   # Temporarily remove any existing priority and prepended date, perform the
   # change (replace/prepend) and re-insert the existing priority and prepended
   # date again.
   cleaninput "for sed"
-  sed -i.bak -e "$item s/^${priority}${prepdate}//" -e "$item s|^.*|${priority}${prepdate}${input}${backref}|" "$TODO_FILE"
+  sed -i.bak -e "$item s/^${originalPriority}${originalPrepdate}//" -e "$item s|^.*|${priority}${prepdate}${input}${backref}|" "$TODO_FILE"
   if [ "$TODOTXT_VERBOSE" -gt 0 ]; then
     getNewtodo "$item"
     case "$action" in