sparse-index: improve advice message instructions

When an on-disk sparse index is expanded to a full one, it could be
due to some worktree state that requires looking at file entries
hidden within sparse tree entries. This can be avoided if the
worktree is cleaned up and some other issues related to the index
state are resolved.

Expand the advice message to include all of these cases, since 'git
sparse-checkout clean' is not currently capable of handling all
cases.

In the future, we may improve the behavior of 'git sparse-checkout
clean' to handle all of the cases.

Helped-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: derrickstolee

sparse-index.c

@@ -32,8 +32,9 @@ int give_advice_on_expansion = 1;
 	"Your working directory likely has contents that are outside of\n"     \
 	"your sparse-checkout patterns. Use 'git sparse-checkout list' to\n"   \
 	"see your sparse-checkout definition and compare it to your working\n" \
-	"directory contents. Running 'git sparse-checkout clean' may assist\n" \
-	"in this cleanup."
+	"directory contents. Cleaning up any merge conflicts or staged\n"      \
+	"changes before running 'git sparse-checkout clean' or 'git\n"         \
+	"sparse-checkout reapply' may assist in this cleanup."
 
 struct modify_index_context {
 	struct index_state *write;