Remove unnecessary delimiter from query for mariadb-client shortcut command

When comparing the string query when processing command name and it's shortcut form, one can obtain the delimiter at the end of the string. This is not needed and we can remove it in order to expect the same format of query/string to be processed by `com_*()` functions and this is now not the case.
-Example

• Shortcut with delimiter

  MariaDB [(none)]> \h;

  Breakpoint 1, com_help (buffer=0x5555555b476b <find_command(char)+297>, line=0x7fffffffd990 "\220\332\377\377\377\177") at /home/anel/GitHub/mariadb/server/src/10.3/client/mysql.cc:3145

  3145 {

  (gdb) p line

  +p line

  $2 = 0x555555a3dfc0 "\\h;"

• Shortcut without delimiter

  MariaDB [(none)]> \h

  Breakpoint 1, com_help (buffer=0x5555555b476b <find_command(char)+297>, line=0x7fffffffd990 "\220\332\377\377\377\177") at /home/anel/GitHub/mariadb/server/src/10.3/client/mysql.cc:3145

  3145 {

  (gdb) n

  +n

  3147 char * help_arg= strchr(line,' '), buff[32], *end;

  (gdb) p line

  +p line

  $3 = 0x555555a3e5b0 "\\h"

• Verbose command

  MariaDB [(none)]> help;

  Breakpoint 1, com_help (buffer=0x5555559f8b98, line=0x555555a3dfc4 ";") at /home/anel/GitHub/mariadb/server/src/10.3/client/mysql.cc:3145

  3145 {

  (gdb) n

  +n

  3147 char * help_arg= strchr(line,' '), buff[32], *end;

  (gdb) p line

  +p line

  $5 = 0x5555559f8b98 "help"

Why is this needed?
MDEV-12192 is parsing the key/value for alias handling, so we need to have deterministic behavior for verbose command alias as well as abbreviation \a.