branch: make the advice to force-deleting a conditional one

The error message we show when the user tries to delete a not fully
merged branch describes the error and gives a hint to the user:

	error: the branch 'foo' is not fully merged.
	If you are sure you want to delete it, run 'git branch -D foo'.

Let's move the hint part so that it is displayed using the advice
machinery:

	error: the branch 'foo' is not fully merged
	hint: If you are sure you want to delete it, run 'git branch -D foo'
	hint: Disable this message with "git config advice.forceDeleteBranch false"

Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

advice.h

@@ -21,6 +21,7 @@ enum advice_type {
 	ADVICE_DETACHED_HEAD,
 	ADVICE_DIVERGING,
 	ADVICE_FETCH_SHOW_FORCED_UPDATES,
+	ADVICE_FORCE_DELETE_BRANCH,
 	ADVICE_GRAFT_FILE_DEPRECATED,
 	ADVICE_IGNORED_HOOK,
 	ADVICE_IMPLICIT_IDENTITY,