gitk: don't highlight files after submodules as submodules
gitk applies submodule highlighting (coloring lines starting with
" >" and " <") when `currdiffsubmod` is not an empty string.
However, it fails to reset `currdiffsubmod` after a submodule diff
ends, so any file diffs following a submodule diff will still be
highlighted as if they were submodule diffs.
There are two problems with the way gitk tries to reset `currdiffsubmod`:
1. The code says `set $currdiffsubmod` instead of `set currdiffsubmod`,
so it actually sets the variable whose name is the submodule path
instead.
2. It tries to do it after the first line in a submodule diff, which
is incorrect, since submodule diffs can contain multiple lines.
Fix this by resetting `currdiffsubmod` when a file diff starts.
Signed-off-by: Роман Донченко <dpb@corrigendum.ru>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>