test-terminal: set TERM=vt100

The point of the test-terminal script is to simulate in the
test scripts an environment where output is going to a real
terminal.

But since test-lib.sh also sets TERM=dumb, the simulation
isn't very realistic. The color code will skip auto-coloring
for TERM=dumb, leading to us liberally sprinkling

  test_terminal env TERM=vt100 git ...

through the test suite to convince the tests to actually
generate colors. Let's set TERM for programs run under
test_terminal, which is one less thing for test-writers to
remember.

In most cases the callers can be simplified, but note there
is one interesting case in t4202. It uses test_terminal to
check the auto-enabling of --decorate, but the expected
output _doesn't_ contain colors (because TERM=dumb
suppresses them). Using TERM=vt100 is closer to what the
real world looks like; adjust the expected output to match.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/test-terminal.perl

@@ -80,6 +80,7 @@ sub copy_stdio {
 if ($#ARGV < 1) {
 	die "usage: test-terminal program args";
 }
+$ENV{TERM} = 'vt100';
 my $master_in = new IO::Pty;
 my $master_out = new IO::Pty;
 my $master_err = new IO::Pty;