Introduce `StringIO.flush/1` function
Support mix help --search PATTERN
Fix missing concatenation operator.
Fix test for mix deps.clean.
Merge pull request #2819 from benjamintanweihao/patch-3

Fix missing concatenation operator.
set null prompt function in dumbterm mode
Merge pull request #2821 from k1complete/shown_prompt_erlsh_in_dumbterm

set null prompt function in dumb term mode
Do not set not shell as we know have a custom prompt
Merge pull request #2807 from pma/support_mix_help_search

Support mix help --search PATTERN
Merge pull request #2793 from lexmag/master

Introduce `StringIO.flush/1` function
Update CHANGELOG