gitk: don't highlight files after submodules as submodules

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>