commit: advertise config --global --edit on guessed identity

When the user has no user-wide configuration file, it's faster to use the
newly introduced config file template than to run two commands to set
user.name and user.email. Advise this to the user.

The old advice is kept if the user already has a configuration file since
the template feature would not trigger in this case.
Signed-off-by: NMatthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/commit.c

@@ -42,7 +42,20 @@ static const char * const builtin_status_usage[] = {
 	NULL
 };
 
-static const char implicit_ident_advice[] =
+static const char implicit_ident_advice_noconfig[] =
+N_("Your name and email address were configured automatically based\n"
+"on your username and hostname. Please check that they are accurate.\n"
+"You can suppress this message by setting them explicitly. Run the\n"
+"following command and follow the instructions in your editor to edit\n"
+"your configuration file:\n"
+"\n"
+"    git config --global --edit\n"
+"\n"
+"After doing this, you may fix the identity used for this commit with:\n"
+"\n"
+"    git commit --amend --reset-author\n");
+
+static const char implicit_ident_advice_config[] =
 N_("Your name and email address were configured automatically based\n"
 "on your username and hostname. Please check that they are accurate.\n"
 "You can suppress this message by setting them explicitly:\n"
@@ -1343,6 +1356,24 @@ int cmd_status(int argc, const char **argv, const char *prefix)
 	return 0;
 }
 
+static const char *implicit_ident_advice(void)
+{
+	char *user_config = NULL;
+	char *xdg_config = NULL;
+	int config_exists;
+
+	home_config_paths(&user_config, &xdg_config, "config");
+	config_exists = file_exists(user_config) || file_exists(xdg_config);
+	free(user_config);
+	free(xdg_config);
+
+	if (config_exists)
+		return _(implicit_ident_advice_config);
+	else
+		return _(implicit_ident_advice_noconfig);
+
+}
+
 static void print_summary(const char *prefix, const unsigned char *sha1,
 			  int initial_commit)
 {
@@ -1374,7 +1405,7 @@ static void print_summary(const char *prefix, const unsigned char *sha1,
 		strbuf_addbuf_percentquote(&format, &committer_ident);
 		if (advice_implicit_identity) {
 			strbuf_addch(&format, '\n');
-			strbuf_addstr(&format, _(implicit_ident_advice));
+			strbuf_addstr(&format, implicit_ident_advice());
 		}
 	}
 	strbuf_release(&author_ident);