gitk: Allow unbalanced quotes/braces in commit headers

gitk: Allow unbalanced quotes/braces in commit headers

When parsing commits, gitk treats the headers of the commit as tcl
lists.  This causes errors if the header contains an unbalanced quote
or open brace.  Splitting the line on spaces allows us to treat it as
a set of words instead of as a tcl list, which prevents errors.

Signed-off-by: Kevin Ballard <kevin@sb.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>