add.c: extract check_path_for_gitlink() from treat_gitlinks() for reuse

Extract the body of the for loop in treat_gitlinks() into a separate
check_path_for_gitlink() function so that it can be reused elsewhere.
This paves the way for a new check-ignore sub-command.

Also document treat_gitlinks().

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

pathspec.h

@@ -3,5 +3,6 @@
 
 extern char *find_pathspecs_matching_against_index(const char **pathspec);
 extern void add_pathspec_matches_against_index(const char **pathspec, char *seen, int specs);
+extern const char *check_path_for_gitlink(const char *path);
 
 #endif /* PATHSPEC_H */