ex-command `ab` missing

# Issue type

- Enhancement request

# Environment

# Emacs version:
GNU Emacs 27.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version 3.22.21) of 2017-09-21

# Operating System:
arch linux
# Evil version:
1.2.12
# Evil installation type:
MELPA
# Graphical/Terminal:
Graphical X
# Tested in a `make emacs` session:
Yes
# Reproduction steps
- Start Emacs
(package-initialize)
(require 'evil)
(evil-mode)
: ab abbrev abbreviation

# Expected behavior
abbreviation added to global-abbrev-table

# Actual behavior
error message:
Unknown command: 'ab'

Start by tracing the ex-command path exercised by `: ab abbrev abbreviation` after `(evil-mode)`, and compare how related commands are handled. Done means the command is accepted and the abbreviation is added to `global-abbrev-table`, with a regression check covering the reproduction.

Written by the indexing model from the issue text.