gitk: Show diff of commits at end of compare-commits output

gitk: Show diff of commits at end of compare-commits output

When comparing a string of commits, when we find two non-merge commits
that differ, we now write the two commits to files and diff the files.
This pulls out the logic for creating a temporary directory from
external_diff into a separate procedure so that the new diffcommits
procedure can use it.

Because the diff command returns an exit status of 1 when the files
differ, and Tcl treats that as an error, this adds catch {} around the
close statements in getblobdiffline.

At present this only removes the temporary files when gitk exits.  It
should remove them when the diff is done.

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