Add match_prev_cmd strategy.

A new suggestion strategy 'match_prev_cmd' is available. This is a bit
more context aware variaton on the default strategy.
The suggestion will be:
- The newest history entry that matches the current prefix, AND
- Whose preceding history entry also matches the previously executed
command.

See src/strategies/match_prev_cmd.zsh for an example.

zsh-autosuggestions.zsh

@@ -330,6 +330,11 @@ _zsh_autosuggest_escape_command_prefix() {
 	echo -E "${1//(#m)[\\()\[\]|*?]/\\$MATCH}"
 }
 
+# Get the previously executed command (hookable for testing)
+_zsh_autosuggest_prev_command() {
+	echo -E "${history[$((HISTCMD-1))]}"
+}
+
 #--------------------------------------------------------------------#
 # Default Suggestion Strategy                                        #
 #--------------------------------------------------------------------#
@@ -347,6 +352,56 @@ _zsh_autosuggest_strategy_default() {
 	echo -E "${history[$histkey]}"
 }
 
+#--------------------------------------------------------------------#
+# Match Previous Command Suggestion Strategy                         #
+#--------------------------------------------------------------------#
+# Suggests the most recent history item that matches the given
+# prefix, and whose preceding history item also matches the most
+# recently executed command.
+#
+# For example, if your have just executed:
+#   pwd
+#   ls foo
+#   ls bar
+#   pwd
+# And then you start typing 'ls', then the suggestion will be 'ls foo',
+# rather than 'ls bar', as your most recently executed command (pwd)
+# was followed by 'ls foo' on it's previous invocation.
+#
+
+_zsh_autosuggest_strategy_match_prev_cmd() {
+	local prefix="$(_zsh_autosuggest_escape_command_prefix "$1")"
+
+	# Get all history event numbers that correspond to history
+	# entries that match pattern $prefix*
+	local history_match_keys
+	history_match_keys=(${(k)history[(R)$prefix*]})
+
+	# By default we use the first history number (most recent history entry)
+	local histkey="${history_match_keys[1]}"
+
+	# Get the previously executed command
+	local prev_cmd="$(_zsh_autosuggest_prev_command)"
+	prev_cmd="$(_zsh_autosuggest_escape_command_prefix $prev_cmd)"
+
+	# Iterate up to the first 200 history event numbers that match $prefix
+	for key in "${(@)history_match_keys[1,200]}"; do
+		# Stop if we ran out of history
+		[[ $key -gt 1 ]] || break
+
+		# See if the history entry preceding the suggestion matches the
+		# previous command, and use it if it does
+		if [[ "${history[$((key - 1))]}" == $prev_cmd ]]; then
+			histkey="$key"
+			break
+		fi
+	done
+
+	# Echo the matched history entry
+	echo -E "$history[$histkey]"
+}
+
+
 #--------------------------------------------------------------------#
 # Start                                                              #
 #--------------------------------------------------------------------#