Merge branch 'fc/advice-diverged-history'

After "git pull" that is configured with pull.rebase=false
merge.ff=only fails due to our end having our own development, give
advice messages to get out of the "Not possible to fast-forward"
state.

* fc/advice-diverged-history:
  advice: add diverging advice for novices

Documentation/config/advice.txt

@@ -136,4 +136,6 @@ advice.*::
 		Advice shown when either linkgit:git-add[1] or linkgit:git-rm[1]
 		is asked to update index entries outside the current sparse
 		checkout.
+	diverging::
+		Advice shown when a fast-forward is not possible.
 --

advice.c

@@ -46,6 +46,7 @@ static struct {
 	[ADVICE_COMMIT_BEFORE_MERGE]			= { "commitBeforeMerge", 1 },
 	[ADVICE_DETACHED_HEAD]				= { "detachedHead", 1 },
 	[ADVICE_SUGGEST_DETACHING_HEAD]			= { "suggestDetachingHead", 1 },
+	[ADVICE_DIVERGING]				= { "diverging", 1 },
 	[ADVICE_FETCH_SHOW_FORCED_UPDATES]		= { "fetchShowForcedUpdates", 1 },
 	[ADVICE_GRAFT_FILE_DEPRECATED]			= { "graftFileDeprecated", 1 },
 	[ADVICE_IGNORED_HOOK]				= { "ignoredHook", 1 },
@@ -219,6 +220,14 @@ void NORETURN die_conclude_merge(void)
 
 void NORETURN die_ff_impossible(void)
 {
+	advise_if_enabled(ADVICE_DIVERGING,
+		_("Diverging branches can't be fast-forwarded, you need to either:\n"
+		"\n"
+		"\tgit merge --no-ff\n"
+		"\n"
+		"or:\n"
+		"\n"
+		"\tgit rebase\n"));
 	die(_("Not possible to fast-forward, aborting."));
 }
 

advice.h

@@ -19,6 +19,7 @@ struct string_list;
 	ADVICE_CHECKOUT_AMBIGUOUS_REMOTE_BRANCH_NAME,
 	ADVICE_COMMIT_BEFORE_MERGE,
 	ADVICE_DETACHED_HEAD,
+	ADVICE_DIVERGING,
 	ADVICE_SUGGEST_DETACHING_HEAD,
 	ADVICE_FETCH_SHOW_FORCED_UPDATES,
 	ADVICE_GRAFT_FILE_DEPRECATED,