add HISTORY_SUBSTRING_SEARCH_PREFIX (#112,#115)

* `HISTORY_SUBSTRING_SEARCH_PREFIX` is a global variable that defines
  how the command history will be searched for your query. If set to a non-empty
  value, only history prefixed by your query will be matched. For example,
  if this variable is empty, `ls` will match `ls -l` and `echo ls`; if it is
  non-empty, `ls` will only match `ls -l`.

Co-authored-by: Xue Qianming <qianmingxue@microsoft.com>

Author: SleepyBag

README.md

@@ -141,6 +141,12 @@ default values only after having loaded this script into your ZSH session.
   value, causes this script to perform a fuzzy search by words, matching in
   given order e.g. `ab c` will match `*ab*c*`
 
+* `HISTORY_SUBSTRING_SEARCH_PREFIX` is a global variable that defines
+  how the command history will be searched for your query. If set to a non-empty
+  value, only history prefixed by your query will be matched. For example,
+  if this variable is empty, `ls` will match `ls -l` and `echo ls`; if it is
+  non-empty, `ls` will only match `ls -l`.
+
 * `HISTORY_SUBSTRING_SEARCH_ENSURE_UNIQUE` is a global variable that defines
   whether all search results returned are _unique_. If set to a non-empty
   value, then only unique search results are presented. This behaviour is off

zsh-history-substring-search.zsh

@@ -48,6 +48,7 @@ typeset -g HISTORY_SUBSTRING_SEARCH_HIGHLIGHT_NOT_FOUND='bg=red,fg=white,bold'
 typeset -g HISTORY_SUBSTRING_SEARCH_GLOBBING_FLAGS='i'
 typeset -g HISTORY_SUBSTRING_SEARCH_ENSURE_UNIQUE=''
 typeset -g HISTORY_SUBSTRING_SEARCH_FUZZY=''
+typeset -g HISTORY_SUBSTRING_SEARCH_PREFIX=''
 
 #-----------------------------------------------------------------------------
 # declare internal global variables
@@ -246,7 +247,8 @@ _history-substring-search-begin() {
     # Escape and join query parts with wildcard character '*' as seperator
     # `(j:CHAR:)` join array to string with CHAR as seperator
     #
-    local search_pattern="*${(j:*:)_history_substring_search_query_parts[@]//(#m)[\][()|\\*?#<>~^]/\\$MATCH}*"
+    local search_pattern="${(j:*:)_history_substring_search_query_parts[@]//(#m)[\][()|\\*?#<>~^]/\\$MATCH}*"
+    test -z "$HISTORY_SUBSTRING_SEARCH_PREFIX" && search_pattern="*$search_pattern"
 
     #
     # Find all occurrences of the search pattern in the history file.