gitk: Fix display of "(...)" for parents/children we haven't drawn

gitk: Fix display of "(...)" for parents/children we haven't drawn

In the commit details window, we were displaying "(...)" for the
headlines of parents and children that haven't been drawn, without
making any attempt to get those headlines.  This adds a call to
getcommit to commit_descriptor so we get those headlines.

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