built-in add -i: do not try to patch/diff an empty list of files

When the user does not select any files to `patch` or `diff`, there is
no need to call `run_add_p()` on them.

Even worse: we _have_ to avoid calling `parse_pathspec()` with an empty
list because that would trigger this error:

	BUG: pathspec.c:557: PATHSPEC_PREFER_CWD requires arguments

So let's avoid doing any work on a list of files that is empty anyway.

This fixes #2466.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

add-interactive.c

@@ -931,7 +931,7 @@ static int run_patch(struct add_i_state *s, const struct pathspec *ps,
 
 	opts->prompt = N_("Patch update");
 	count = list_and_choose(s, files, opts);
-	if (count >= 0) {
+	if (count > 0) {
 		struct argv_array args = ARGV_ARRAY_INIT;
 		struct pathspec ps_selected = { 0 };
 
@@ -972,7 +972,7 @@ static int run_diff(struct add_i_state *s, const struct pathspec *ps,
 	opts->flags = IMMEDIATE;
 	count = list_and_choose(s, files, opts);
 	opts->flags = 0;
-	if (count >= 0) {
+	if (count > 0) {
 		struct argv_array args = ARGV_ARRAY_INIT;
 
 		argv_array_pushl(&args, "git", "diff", "-p", "--cached",