Return from user prompt without requiring Enter (#354)

* Tests: Add coverage for del / move without -f, but with prompting

Supplying the user confirmation via "yes".

* Cosmetics: Align inconsistent spacing for before (y/n) prompt

* Refactoring: Extract confirm() function

The user confirmation query had been duplicated (once) in the code.

* Refactoring: confirm(): Leave early if forced

* Return from user prompt without requiring Enter

By just reading a single character (y for yes, anything else: no).



* Tests: Ensure that only a single "y" concludes the confirmation

By switching from "yes" (that endlessly prints newline-separated "y"s) to "printf y".

* t1800-del: Add coverage for negative confirmation

Negative means "anything but y", so "n", "x", and Enter all apply.

* Cosmetics: Add trailing space after (y/n) prompt

So that the user's typed answer is not recorded directly after it, but with separation: "Foo? (y/n) y" instead of "Foo? (y/n)y".

*Compatibility: "read -N 1" is only available in Bash 4.1+

Mac OS still ships with Bash 3.2 :-( Fall back to the original prompting that requires conclusion via Enter then.
Note: Even though the tests use "printf y", this still gets accepted, as there'll be EOF after that. In real use (when stdin from the terminal stays open), a concluding Enter is mandatory, though.

Closes #152

tests/t1800-del.sh

@@ -4,6 +4,8 @@ test_description='basic del functionality
 '
 . ./test-lib.sh
 
+SPACE=' '
+
 test_todo_session 'del usage' <<EOF
 >>> todo.sh del B
 usage: todo.sh del ITEM# [TERM]
@@ -44,6 +46,50 @@ TODO: 1 deleted.
 TODO: 2 of 2 tasks shown
 EOF
 
+cat > todo.txt <<EOF
+(B) smell the uppercase Roses +flowers @outside
+(A) notice the sunflowers
+stop
+EOF
+test_todo_session 'del with confirmation' <<EOF
+>>> todo.sh -p list
+2 (A) notice the sunflowers
+1 (B) smell the uppercase Roses +flowers @outside
+3 stop
+--
+TODO: 3 of 3 tasks shown
+
+>>> printf n | todo.sh del 1
+Delete '(B) smell the uppercase Roses +flowers @outside'? (y/n)$SPACE
+TODO: No tasks were deleted.
+
+>>> todo.sh -p list
+2 (A) notice the sunflowers
+1 (B) smell the uppercase Roses +flowers @outside
+3 stop
+--
+TODO: 3 of 3 tasks shown
+
+>>> printf x | todo.sh del 1
+Delete '(B) smell the uppercase Roses +flowers @outside'? (y/n)$SPACE
+TODO: No tasks were deleted.
+
+>>> echo | todo.sh del 1
+Delete '(B) smell the uppercase Roses +flowers @outside'? (y/n)$SPACE
+TODO: No tasks were deleted.
+
+>>> printf y | todo.sh del 1
+Delete '(B) smell the uppercase Roses +flowers @outside'? (y/n)$SPACE
+1 (B) smell the uppercase Roses +flowers @outside
+TODO: 1 deleted.
+
+>>> todo.sh -p list
+2 (A) notice the sunflowers
+3 stop
+--
+TODO: 2 of 2 tasks shown
+EOF
+
 cat > todo.txt <<EOF
 (B) smell the uppercase Roses +flowers @outside
 (A) notice the sunflowers