gitk: Index line[hnd]tag arrays by id rather than row number
gitk: Index line[hnd]tag arrays by id rather than row number

This simplifies things a bit and is better because ids are stable
but row numbers aren't.  It also means we can avoid one [rowofcommit]
call.

Signed-off-by: Paul Mackerras <19a0ba370c443ba08d20b5061586430ab449ee8c@samba.org>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git