git-am: Add colon before the subject that is printed out as being applied

git-am: Add colon before the subject that is printed out as being applied

git-am output can be confusing, because the subject of the applied
patch can look like the rest of a sentence starting with "Applying".
The added colon should make this clearer.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>