refactor handling of "other" files in ls-files and status

When the "git status" display code was originally converted
to C, we copied the code from ls-files to discover whether a
pathname returned by read_directory was an "other", or
untracked, file.

Much later, 5698454e updated the code in ls-files to handle
some new cases caused by gitlinks.  This left the code in
wt-status.c broken: it would display submodule directories
as untracked directories. Nobody noticed until now, however,
because unless status.showUntrackedFiles was set to "all",
submodule directories were not actually reported by
read_directory. So the bug was only triggered in the
presence of a submodule _and_ this config option.

This patch pulls the ls-files code into a new function,
cache_name_is_other, and uses it in both places. This should
leave the ls-files functionality the same and fix the bug
in status.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

read-cache.c

@@ -1480,3 +1480,31 @@ int read_index_unmerged(struct index_state *istate)
 	istate->cache_nr = dst - istate->cache;
 	return !!last;
 }
+
+/*
+ * Returns 1 if the path is an "other" path with respect to
+ * the index; that is, the path is not mentioned in the index at all,
+ * either as a file, a directory with some files in the index,
+ * or as an unmerged entry.
+ *
+ * We helpfully remove a trailing "/" from directories so that
+ * the output of read_directory can be used as-is.
+ */
+int index_name_is_other(const struct index_state *istate, const char *name,
+		int namelen)
+{
+	int pos;
+	if (namelen && name[namelen - 1] == '/')
+		namelen--;
+	pos = index_name_pos(istate, name, namelen);
+	if (0 <= pos)
+		return 0;	/* exact match */
+	pos = -pos - 1;
+	if (pos < istate->cache_nr) {
+		struct cache_entry *ce = istate->cache[pos];
+		if (ce_namelen(ce) == namelen &&
+		    !memcmp(ce->name, name, namelen))
+			return 0; /* Yup, this one exists unmerged */
+	}
+	return 1;
+}