add: warn when asked to update SKIP_WORKTREE entries

`git add` already refrains from updating SKIP_WORKTREE entries, but it
silently exits with zero code when it is asked to do so. Instead, let's
warn the user and display a hint on how to update these entries.

Note that we only warn the user whey they give a pathspec item that
matches no eligible path for updating, but it does match one or more
SKIP_WORKTREE entries. A warning was chosen over erroring out right away
to reproduce the same behavior `add` already exhibits with ignored
files. This also allow users to continue their workflow without having
to invoke `add` again with only the eligible paths (as those will have
already been added).

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 4 insertions, 0 deletions

diff --git a/advice.h b/advice.h
index bc2432980a..bd26c385d0 100644
--- a/advice.h
+++ b/advice.h
@@ -3,6 +3,8 @@
 
 #include "git-compat-util.h"
 
+struct string_list;
+
 extern int advice_fetch_show_forced_updates;
 extern int advice_push_update_rejected;
 extern int advice_push_non_ff_current;
@@ -71,6 +73,7 @@ extern int advice_add_empty_pathspec;
 	ADVICE_STATUS_HINTS,
 	ADVICE_STATUS_U_OPTION,
 	ADVICE_SUBMODULE_ALTERNATE_ERROR_STRATEGY_DIE,
+	ADVICE_UPDATE_SPARSE_PATH,
 	ADVICE_WAITING_FOR_EDITOR,
 };
 
@@ -92,6 +95,7 @@ void advise_if_enabled(enum advice_type type, const char *advice, ...);
 int error_resolve_conflict(const char *me);
 void NORETURN die_resolve_conflict(const char *me);
 void NORETURN die_conclude_merge(void);
+void advise_on_updating_sparse_paths(struct string_list *pathspec_list);
 void detach_advice(const char *new_name);
 
 #endif /* ADVICE_H */