Fix machine-parseability of 'git log --source'

Fix machine-parseability of 'git log --source'

The space between the commit and the source attribute is not easily
machine-parseable: if we combine --source with --parents and give a SHA1
as a starting point, it's unnecessarily hard to see where the list of
parents ends and the source decoration begins.

Example:
	git show --parents --source $(git rev-list HEAD)

which is admittedly contrived, but can easily happen in scripting.

So use a <tab> instead of a space as the source separator.

The other decorations didn't have this issue, because they were surrounded
by parenthesis, so it's obvious that they aren't parent SHA1's.

It so happens that _visually_ this makes no difference for "git log
--source", since "commit <40-char SHA1>" is 47 characters, so both a space
and a <tab> will end up showing as a single commit. Of course, with
'--pretty=oneline' or '--parents' or '--abbrev-commit' you'll see the
difference.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>