Emacs M-x shell does not work git-completion.bash

Based on the article below, I set git-completion.bash and git-prompt.sh.

Good news for you like "I don't know how to supplement Git" and "I use git status 100 times a day" itgit-completion and git-prompt /

Same ~/.bashrc shared terminal bash with shell buffer launched with M-x shell in Emacs, but git-prompt.sh was enabled and the branch name could be displayed at the prompt.

However, for some reason, git-completion.bash is enabled in the terminal, but not in the Emacs shell.
How can I enable it with Emacs?

The following is the environment.

Windows 10 is running WSL1 and starting Ubuntu.

$cat/etc/lsb-release
DISTRIB_ID = Ubuntu
DISTRIB_RELEASE=20.04
DISTRIB_CODENAME=focal
DISTRIB_DESCRIPTION="Ubuntu 20.04.2 LTS"

$ emacs-version
GNU Emacs 26.3

Thank you for your cooperation.