Make user completion file configurable, disable in tests

No longer need to override $HOME with this. Doing that made some tilde
expansion tests fail, because the overridden value affected
eval-expansion of only plain ~, not ~user (for the same user).

Fixes #128

bash_completion

@@ -2078,8 +2078,11 @@ fi
 unset compat_dir i _blacklist_glob
 
 # source user completion file
-[[ ${BASH_SOURCE[0]} != ~/.bash_completion && -r ~/.bash_completion ]] \
-    && . ~/.bash_completion
+user_completion=${BASH_COMPLETION_USER_FILE:-~/.bash_completion}
+[[ ${BASH_SOURCE[0]} != $user_completion && -r $user_completion ]] \
+    && . $user_completion
+unset user_completion
+
 unset -f have
 unset have
 

doc/bash_completion.txt

@@ -4,10 +4,11 @@ Bash completion
 Configuration files
 -------------------
 
-*~/.bash_completion*::
+*$BASH_COMPLETION_USER_FILE*::
     Sourced late by bash_completion, pretty much after everything else.
     Use this file for example to load additional completions, and to remove
-    and override ones installed by bash_completion.
+    and override ones installed by bash_completion. Defaults to
+    `~/.bash_completion` if unset or null.
 
 *$XDG_CONFIG_HOME/bash_completion*::
     Sourced by the bash_completion.sh profile.d script.  This file is

test/config/bashrc

@@ -24,7 +24,7 @@ export PATH=$PATH:/sbin:/usr/sbin:/usr/local/sbin
     # ...as well as games on some systems not in PATH by default:
 export PATH=$PATH:/usr/games:/usr/local/games
     # For clean test state, avoid sourcing user's ~/.bash_completion
-export HOME=$SRCDIR/config
+export BASH_COMPLETION_USER_FILE=/dev/null
     # ...and avoid stuff in BASH_COMPLETION_USER_DIR overriding in-tree
     # completions. The user dir is first in the lookup path, so this should also
     # give precedence to the in-tree "completions" dir over other ones, e.g.