Improve merge function

Fix some data loss when redirecting a large merged history file directly
to ZSH_HISTORY_FILE, use a temporary file instead.
Remove blank lines in ZSH_HISTORY_FILE with sed.

history-sync.plugin.zsh

@@ -27,6 +27,7 @@ ZSH_HISTORY_FILE="${ZSH_HISTORY_FILE:-${HOME}/${ZSH_HISTORY_FILE_NAME}}"
 ZSH_HISTORY_FILE_ENC_NAME="${ZSH_HISTORY_FILE_ENC_NAME:-zsh_history}"
 ZSH_HISTORY_FILE_ENC="${ZSH_HISTORY_FILE_ENC:-${ZSH_HISTORY_PROJ}/${ZSH_HISTORY_FILE_ENC_NAME}}"
 ZSH_HISTORY_FILE_DECRYPT_NAME="${ZSH_HISTORY_FILE_DECRYPT_NAME:-zsh_history_decrypted}"
+ZSH_HISTORY_FILE_MERGED_NAME="${ZSH_HISTORY_FILE_MERGED_NAME:-zsh_history_merged}"
 ZSH_HISTORY_COMMIT_MSG="${ZSH_HISTORY_COMMIT_MSG:-latest $(date)}"
 
 function _print_git_error_msg() {
@@ -159,13 +160,15 @@ function history_sync_pull() {
     [[ -o extendedhistory ]] && _squash_multiline_commands_in_files
 
     # Merge
-    cat "$ZSH_HISTORY_FILE" "$ZSH_HISTORY_FILE_DECRYPT_NAME" | awk '/:[0-9]/ { if(s) { print s } s=$0 } !/:[0-9]/ { s=s"\n"$0 } END { print s }' | LC_ALL=C sort -u > "$ZSH_HISTORY_FILE"
+    cat "$ZSH_HISTORY_FILE" "$ZSH_HISTORY_FILE_DECRYPT_NAME" | awk '/:[0-9]/ { if(s) { print s } s=$0 } !/:[0-9]/ { s=s"\n"$0 } END { print s }' | LC_ALL=C sort -u > "$ZSH_HISTORY_FILE_MERGED_NAME"
+    mv "$ZSH_HISTORY_FILE_MERGED_NAME" "$ZSH_HISTORY_FILE"
     rm  "$ZSH_HISTORY_FILE_DECRYPT_NAME"
     cd  "$DIR"
 
     # Check if EXTENDED_HISTORY is enabled, and if so, restore multi-line
     # commands in the local history file
     [[ -o extendedhistory ]] && _restore_multiline_commands_in_file
+    sed -i '/^$/d' "$ZSH_HISTORY_FILE"
 }
 
 # Encrypt and push current history to master