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, 15 insertions, 0 deletions

diff --git a/pathspec.c b/pathspec.c
index 6d502e64e5..8247a65ec8 100644
--- a/pathspec.c
+++ b/pathspec.c
@@ -62,6 +62,21 @@ char *find_pathspecs_matching_against_index(const struct pathspec *pathspec,
 	return seen;
 }
 
+char *find_pathspecs_matching_skip_worktree(const struct pathspec *pathspec)
+{
+	struct index_state *istate = the_repository->index;
+	char *seen = xcalloc(pathspec->nr, 1);
+	int i;
+
+	for (i = 0; i < istate->cache_nr; i++) {
+		struct cache_entry *ce = istate->cache[i];
+		if (ce_skip_worktree(ce))
+		    ce_path_match(istate, ce, pathspec, seen);
+	}
+
+	return seen;
+}
+
 /*
  * Magic pathspec
  *