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>

prompt.h

@@ -6,4 +6,6 @@
 
 char *git_prompt(const char *prompt, int flags);
 
+int git_read_line_interactively(struct strbuf *line);
+
 #endif /* PROMPT_H */