interactive: refactor code asking the user for interactive input
There are quite a few code locations (e.g. `git clean --interactive`)
where Git asks the user for an answer. In preparation for fixing a bug
shared by all of them, and also to DRY up the code, let's refactor it.
Please note that most of these callers trimmed white-space both at the
beginning and at the end of the answer, instead of trimming only the
end (as the caller in `add-patch.c` does).
Therefore, technically speaking, we change behavior in this patch. At
the same time, it can be argued that this is actually a bug fix.
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de> Signed-off-by: Junio C Hamano <gitster@pobox.com>