add.c: move pathspec matchers into new pathspec.c for reuse

Extract the following functions from builtin/add.c to pathspec.c, in
preparation for reuse by a new git check-ignore command:

  - fill_pathspec_matches()
  - find_used_pathspec()

The functions being extracted are not changed in any way, except
removal of the 'static' qualifier.

Also add comments documenting these newly public functions,
including clarifications that they operate on the index.

Signed-off-by: Adam Spiers <git@adamspiers.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

pathspec.h

@@ -0,0 +1,7 @@
+#ifndef PATHSPEC_H
+#define PATHSPEC_H
+
+extern char *find_used_pathspec(const char **pathspec);
+extern void fill_pathspec_matches(const char **pathspec, char *seen, int specs);
+
+#endif /* PATHSPEC_H */