Many strings we need to convert to UTF-8 (paths, even log messages) contain large ASCII chars only sections.  Add utility functions to very efficiently skip those sections at the begin of the strings.
Many strings we need to convert to UTF-8 (paths, even log messages)
contain large ASCII chars only sections.  Add utility functions
to very efficiently skip those sections at the begin of the strings.

* subversion/libsvn_subr/utf_validate.c
  (first_non_ascii_char,
   first_non_ascii_char_cstring): new utility functions
  (svn_utf__last_valid,
   svn_utf__cstring_is_valid,
   svn_utf__is_valid,
   svn_utf__last_valid2): use the new functions to quickly
   skip ASCII-only sections at the head of the strings

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion