gitk: Fix some bugs introduced by speedup changes

Commits that weren't read from git-rev-list, i.e. the ones displayed
with an open circle, were displayed incorrectly: the headline was
null if there was only one line, and the commit comment was put all
on one line.  Also, the terminal commits weren't displayed when -r
was used.

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