gitk: Fix a couple of bugs

gitk: Fix a couple of bugs

insertrow and removerow were trying to adjust rowidlist, rowisopt
and rowfinal even if the row where we're inserting/deleting stuff
hasn't been laid out yet, which resulted in Tcl errors.  This fixes
that.

Also we weren't deleting the link$linknum tag in appendwithlinks,
which resulted in SHA1 IDs in the body of a commit message sometimes
getting shown in blue with underlining when they shouldn't.

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