advice: add diverging advice for novices

The user might not necessarily know why ff only was configured, maybe an
admin did it, or the installer (Git for Windows), or perhaps they just
followed some online advice.

This can happen not only on pull.ff=only, but merge.ff=only too.

Even worse if the user has configured pull.rebase=false and
merge.ff=only, because in those cases a diverging merge will constantly
keep failing. There's no trivial way to get out of this other than
`git merge --no-ff`.

Let's not assume our users are experts in git who completely understand
all their configurations.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Acked-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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

@@ -44,6 +44,7 @@ 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 },
@@ -217,6 +218,14 @@ 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

@@ -21,6 +21,7 @@ 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,