builtin-add: refactor the meat of interactive_add()

This moves the call setup for 'git add--interactive' to a separate
function, as other users will call it without running
validate_pathspec() first.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.h

@@ -137,6 +137,8 @@ int is_descendant_of(struct commit *, struct commit_list *);
 int in_merge_bases(struct commit *, struct commit **, int);
 
 extern int interactive_add(int argc, const char **argv, const char *prefix);
+extern int run_add_interactive(const char *revision, const char *patch_mode,
+			       const char **pathspec);
 
 static inline int single_parent(struct commit *commit)
 {