test-lib-functions: keep user's debugger config files and TERM in 'debug'

test-lib-functions: keep user's debugger config files and TERM in 'debug'

The 'debug' function in test-lib-functions.sh is used to invoke a
debugger at a specific line in a test. It inherits the value of HOME and
TERM set by 'test-lib.sh': HOME="$TRASH_DIRECTORY" and TERM=dumb.

Changing the value of HOME means that any customization configured in a
developers' debugger configuration file (like $HOME/.gdbinit or
$HOME/.lldbinit) are not available in the debugger invoked by
'test_pause'.

Changing the value of TERM to 'dumb' means that colored output
is disabled in the debugger.

To make the debugging experience with 'debug' more pleasant, leverage
the variable USER_HOME, added in the previous commit, to copy a
developer's ~/.gdbinit and ~/.lldbinit to the test HOME. We do not set
HOME to USER_HOME as in 'test_pause' to avoid user configuration in
$USER_HOME/.gitconfig from interfering with the command being debugged.

Also, add a flag to launch the debugger with the original value of
TERM, and add the same warning as for 'test_pause'.

Helped-by: Carlo Marcelo Arenas Belón <carenas@gmail.com>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Acked-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>