i18n: use locale insensitive format for datetimes as intermediate representation (issue3398) on some non "en" locale environments, "hg convert" is aborted, because "util.parsedate()" fails. it fails in "memctx.__init__()" called by "putcommit()" of "convert". in "hg convert", datetimes gotten from source repository are usually formatted by "util.datestr()" with default format "%a %b %d %H:%M:%S %Y %1%2". but on some environments, "%a" and "%b" may cause locale sensitive string, and such string may cause parse error in "util.parsedate()". this path uses "%Y-%m-%d %H:%M:%S %1%2" as intermediate representation format for datetimes, because it consists only of locale insensitive elements. datetimes in above format are only used for passing them from conversion logic to memctx object, so it doesn't have to be formatted by locale sensitive one. this patch just avoids locale sensitivity problem of "datestr()" and "parsedate()" combintion.