gitk: fix the context menu not appearing in the presence of submodule diffs

Currently, submodule diffs can cause the diff context menu to fail to appear because of a couple bugs in parseblobdiffline: * it appends the submodule name to ctext_file_lines instead of a line number, which breaks the binary search in find_ctext_fileinfo; * it can desynchronize ctext_file_names and ctext_file_lines by appending to the former but not the latter, which also breaks find_ctext_fileinfo. Fix both of these. Note: a side effect of this patch is that the context menu also starts appearing when you right-click on submodule diffs (and not just regular diffs). The menu is non-functional in this case, though, since you can't run blame on submodules. Signed-off-by: Роман Донченко <dpb@corrigendum.ru> Signed-off-by: Paul Mackerras <paulus@ozlabs.org>