Fix handling of format string text characters in to_timestamp()/to_date()

Fix handling of format string text characters in to_timestamp()/to_date()

cf984672 introduced improvement of handling of spaces and separators in
to_timestamp()/to_date() functions.  In particular, now we're skipping spaces
both before and after fields.  That may cause format string text character to
consume part of field in the situations, when it didn't happen before cf984672.
This commit cause format string text character consume input string characters
only when since previous field (or string beginning) number of skipped input
string characters is not greater than number of corresponding format string
characters (that is we didn't skip any extra characters in input string).