gitk: Fix compare-commits function when we have local changes

gitk: Fix compare-commits function when we have local changes

This fixes a bug in the compare-commits function added in commit
010509f2 ("gitk: Add a command to compare two strings of commits")
where gitk would show an error dialog if the comparison of commits
got to a fake commit (one showing local changes).  It extends
getpatchid to handle these fake commits by using [diffcmd] to get
the git diff command variant to use, and also handles the situation
where an error occurs.

Now that we can have the fake commit IDs showing up, which are
00..00 and 00..01, the short ID is ambiguous.  To make sure the links
point to the right commit, this adds a new [appendshortlink] procedure
which takes the full link destination, and uses that rather than
appendwithlinks.

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