Add bash-git-prompt in order to have a more descriptive prompt when working with repositories.

Also, update documentation for bash-git-prompt usage. Make sure initial value for usage is false.

env-example

@@ -158,7 +158,7 @@ WORKSPACE_INSTALL_FFMPEG=false
 WORKSPACE_INSTALL_GNU_PARALLEL=false
 WORKSPACE_INSTALL_AST=true
 WORKSPACE_AST_VERSION=1.0.3
-WORKSPACE_INSTALL_GIT_PROMPT=true
+WORKSPACE_INSTALL_GIT_PROMPT=false
 
 ### PHP_FPM ###############################################