test-lib-functions: optionally keep HOME, TERM and SHELL in 'test_pause'

The 'test_pause' function, which is designed to help interactive
debugging and exploration of tests, currently inherits the value of HOME
and TERM set by 'test-lib.sh': HOME="$TRASH_DIRECTORY" and TERM=dumb. It
also invokes the shell defined by TEST_SHELL_PATH, which defaults to
/bin/sh (through SHELL_PATH).

Changing the value of HOME means that any customization configured in a
developers' shell startup files and any Git aliases defined in their
global Git configuration file are not available in the shell invoked by
'test_pause'.

Changing the value of TERM to 'dumb' means that colored output
is disabled for all commands in that shell.

Using /bin/sh as the shell invoked by 'test_pause' is not ideal since
some platforms (i.e. Debian and derivatives) use Dash as /bin/sh, and
this shell is usually compiled without readline support, which makes for
a poor interactive command line experience.

To make the interactive command line experience in the shell invoked by
'test_pause' more pleasant, save the values of HOME and TERM in
USER_HOME and USER_TERM before changing them in test-lib.sh, and add
options to 'test_pause' to optionally use these variables to invoke the
shell. Also add an option to invoke SHELL instead of TEST_SHELL_PATH, so
that developer's interactive shell is used.

We use options instead of changing the behaviour unconditionally since
these three variables can slightly change command behaviour. Moreover,
using the original HOME means commands could overwrite files in a user's
home directory. Be explicit about these caveats in the new 'Usage'
section in test-lib-functions.sh.

Finally, add '[options]' to the test_pause synopsys in t/README, and
mention that the full list of helper functions and their options can be
found in test-lib-functions.sh.

Helped-by: Elijah Newren <newren@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>

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