Consider delimiter length in dir truncate right

Truncating the path from the right now takes in account the delimiter
length, so that directories with names shorter than truncated name +
delimiter are displayed properly.

For example, if SHORTEN_DIR_LENGTH is 4 and the delimiter is "..",
"../tests/.." and "../custom/.." are not incorrectly "truncated" to
"../test../.." and "../cust../..", both of which are longer or the same
length as the original.

functions/utilities.zsh

@@ -204,5 +204,7 @@ function segmentShouldBeJoined() {
 # Given a directory path, truncate it according to the settings for
 # `truncate_from_right`
 function truncatePathFromRight() {
-  echo $1 | sed $SED_EXTENDED_REGEX_PARAMETER "s/([^/]{$POWERLEVEL9K_SHORTEN_DIR_LENGTH})[^/]+\//\1$POWERLEVEL9K_SHORTEN_DELIMITER\//g"
+  local delim_len=${#POWERLEVEL9K_SHORTEN_DELIMITER}
+  echo $1 | sed $SED_EXTENDED_REGEX_PARAMETER \
+ "s@(([^/]{$((POWERLEVEL9K_SHORTEN_DIR_LENGTH))})([^/]{$delim_len}))[^/]+/@\2$POWERLEVEL9K_SHORTEN_DELIMITER/@g"
 }