Improve hint for diverging branches.

Added a description of what the offered options will do and also
offered a 3rd during pull - a hard reset.
This option should be helpful for the new users that accidentally
committed into the wrong branch.

There is some danger because it's semi-destructive, but so are
other options offered if user doesn't know the commands,
additionally I think "To discard your local changes" wording
describes this clearly enough.

Signed-off-by: Konstantin Pereiaslov <perk11@perk11.info>

Documentation/config/advice.txt

@@ -137,7 +137,8 @@ advice.*::
 		is asked to update index entries outside the current sparse
 		checkout.
 	diverging::
-		Advice shown when a fast-forward is not possible.
+		Advice shown when a fast-forward is not possible during merge
+		or pull operation.
 	worktreeAddOrphan::
 		Advice shown when a user tries to create a worktree from an
 		invalid reference, to instruct how to create a new orphan

advice.c

@@ -220,19 +220,38 @@ void NORETURN die_conclude_merge(void)
 	die(_("Exiting because of unfinished merge."));
 }
 
-void NORETURN die_ff_impossible(void)
+void NORETURN die_ff_impossible_during_merge(void)
 {
 	advise_if_enabled(ADVICE_DIVERGING,
-		_("Diverging branches can't be fast-forwarded, you need to either:\n"
+		_("Diverging branches can't be fast-forwarded."
+		"Consider the following options:\n"
 		"\n"
+		"To merge remote changes into your branch:\n"
 		"\tgit merge --no-ff\n"
 		"\n"
-		"or:\n"
-		"\n"
+		"To apply your changes on top of remote changes:\n"
 		"\tgit rebase\n"));
 	die(_("Not possible to fast-forward, aborting."));
 }
 
+void NORETURN die_ff_impossible_during_pull(const char *upstream_branch_spec)
+{
+	advise_if_enabled(ADVICE_DIVERGING,
+			  _("Diverging branches can't be fast-forwarded. "
+			    "Consider the following options:\n"
+			    "\n"
+			    "To merge remote changes into your branch:\n"
+			    "\tgit merge --no-ff\n"
+			    "\n"
+			    "To apply your changes on top of remote changes:\n"
+			    "\tgit rebase\n"
+			    "\n"
+			    "To discard your local changes and apply the remote changes:\n"
+			    "\tgit reset --hard %s\n"), upstream_branch_spec);
+	die(_("Not possible to fast-forward, aborting."));
+}
+
+
 void advise_on_updating_sparse_paths(struct string_list *pathspec_list)
 {
 	struct string_list_item *item;

advice.h

@@ -71,7 +71,8 @@ void advise_if_enabled(enum advice_type type, const char *advice, ...);
 int error_resolve_conflict(const char *me);
 void NORETURN die_resolve_conflict(const char *me);
 void NORETURN die_conclude_merge(void);
-void NORETURN die_ff_impossible(void);
+void NORETURN die_ff_impossible_during_merge(void);
+void NORETURN die_ff_impossible_during_pull(const char *upstream_branch_spec);
 void advise_on_updating_sparse_paths(struct string_list *pathspec_list);
 void detach_advice(const char *new_name);
 void advise_on_moving_dirty_path(struct string_list *pathspec_list);

builtin/merge.c

@@ -1675,7 +1675,7 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 	}
 
 	if (fast_forward == FF_ONLY)
-		die_ff_impossible();
+		die_ff_impossible_during_merge();
 
 	if (autostash)
 		create_autostash(the_repository,

builtin/pull.c

@@ -765,6 +765,20 @@ static const char *get_tracking_branch(const char *remote, const char *refspec)
 	refspec_item_clear(&spec);
 	return merge_branch;
 }
+/**
+ * Returns the branch the pull is performed from.
+ * If remote is NULL or refspec is NULL, configured upstream remote of the current branch is used.
+ * If refspec is NULL, the current configured upstream branch is used.
+ */
+static const char *get_pull_branch(const char *remote, const char *refspec)
+{
+	if (refspec == NULL || remote == NULL)
+	{
+		return get_upstream_branch(remote);
+	}
+
+	return get_tracking_branch(remote, refspec);
+}
 
 /**
  * Given the repo and refspecs, sets fork_point to the point at which the
@@ -1112,8 +1126,10 @@ int cmd_pull(int argc, const char **argv, const char *prefix)
 
 	/* ff-only takes precedence over rebase */
 	if (opt_ff && !strcmp(opt_ff, "--ff-only")) {
-		if (divergent)
-			die_ff_impossible();
+		if (divergent) {
+			const char* pull_branch_spec = get_pull_branch(repo, *refspecs);
+			die_ff_impossible_during_pull( pull_branch_spec);
+		}
 		opt_rebase = REBASE_FALSE;
 	}
 	/* If no action specified and we can't fast forward, then warn. */