repack: make '--quiet' disable progress

While testing some ideas in 'git repack', I ran it with '--quiet' and
discovered that some progress output was still shown. Specifically, the
output for writing the multi-pack-index showed the progress.

The 'show_progress' variable in cmd_repack() is initialized with
isatty(2) and is not modified at all by the '--quiet' flag. The
'--quiet' flag modifies the po_args.quiet option which is translated
into a '--quiet' flag for the 'git pack-objects' child process. However,
'show_progress' is used to directly send progress information to the
multi-pack-index writing logic which does not use a child process.

The fix here is to modify 'show_progress' to be false if po_opts.quiet
is true, and isatty(2) otherwise. This new expectation simplifies a
later condition that checks both.

Update the documentation to make it clear that '-q' will disable all
progress in addition to ensuring the 'git pack-objects' child process
will receive the flag.

Use 'test_terminal' to check that this works to get around the isatty(2)
check.

Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-repack.txt

@@ -76,8 +76,9 @@ to the new separate pack will be written.
 	linkgit:git-pack-objects[1].
 
 -q::
-	Pass the `-q` option to 'git pack-objects'. See
-	linkgit:git-pack-objects[1].
+--quiet::
+	Show no progress over the standard error stream and pass the `-q`
+	option to 'git pack-objects'. See linkgit:git-pack-objects[1].
 
 -n::
 	Do not update the server information with