Use %Y-%m-%d %H:%M:%S format, update tests

Update history docs.

Note - the omission of a mention of timezone was intentional. These were recorded as naive timestamps lacking timezone information in the first place.

doc_src/history.txt

@@ -34,7 +34,7 @@ The following options are available:
 
 - `-p` or `--prefix` searches or deletes items in the history that begin with the specified text string. This is not currently supported by the `--delete` flag.
 
-- `-t` or `--with-time` outputs the date and time history items were recorded at before each history entry, formatted for the configured locale. See environ(7) for more information.
+- `-t` or `--with-time` outputs the date and time ("%Y-%m-%d %H:%M:%S") history items were recorded at on a line starting with "#" before each history entry. 
 
 \subsection history-examples Example
 

src/history.cpp

@@ -1402,8 +1402,8 @@ static bool format_history_record(const history_item_t &item, const bool with_ti
         const time_t seconds = item.timestamp();
         struct tm timestamp;
         if (!localtime_r(&seconds, &timestamp)) return false;
-        wchar_t timestamp_string[64];
-        if (std::wcsftime(timestamp_string, 63, L"# %c:\n", &timestamp) == 0) return false;
+        wchar_t timestamp_string[24];
+        if (std::wcsftime(timestamp_string, 23, L"# %Y-%m-%d %H:%M:%S\n", &timestamp) == 0) return false;
         streams.out.append(timestamp_string);
     }
     streams.out.append(item.str());

tests/history.expect

@@ -78,7 +78,7 @@ expect_prompt -re {\r\necho start1.*\r\necho start2} {
 # ==========
 # Verify implicit searching with a request for timestamps includes the timestamps.
 send "history -t echo start\r"
-expect_prompt -re {\r\n\d\d\d\d-\d\d-\d\d \d\d:\d\d:\d\d  echo start1; builtin history;.*\r\n\d\d\d\d-\d\d-\d\d \d\d:\d\d:\d\d  echo start2; builtin history} {
+expect_prompt -re {\r\n# \d\d\d\d-\d\d-\d\d \d\d:\d\d:\d\d\r\necho start1; builtin history;.*\r\n# \d\d\d\d-\d\d-\d\d \d\d:\d\d:\d\d\r\necho start2; builtin history} {
     puts "history function implicit search with timestamps succeeded"
 } unmatched {
     puts stderr "history function implicit search with timestamps failed"