submodule: display summary header in bold

submodule: display summary header in bold

Currently, 'git diff --submodule' displays output with a bold diff
header for non-submodules.  So this part is in bold:

    diff --git a/file1 b/file1
    index 30b2f6c..2638038 100644
    --- a/file1
    +++ b/file1

For submodules, the header looks like this:

    Submodule submodule1 012b072..248d0fd:

Unfortunately, it's easy to miss in the output because it's not bold.
Change this.

Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Acked-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>