gitk: Cope with commit messages with carriage-returns and initial blank lines

gitk: Cope with commit messages with carriage-returns and initial blank lines

In some repositories imported from other systems we can get carriage
return characters in the commit message, which leads to a multi-line
headline being displayed in the summary window, which looks bad.
Also some commit messages start with one or more blank lines, which
leads to an empty headline.  This fixes these problems.

Signed-off-by: Paul Mackerras <paulus@samba.org>