[PATCH 0 of 1] files inside repositories are now not reported as unknown in specific cases

[Oleg Stepanov]

Mercurial used to report files residing in subrepositories as unknown. This happened when hg was requested statuses of specific files
and the alphabetically first subrepository didn't contain any of the specified files. The bug was introduces when dirstate.walk
was refactored from O(M*N) algorithm to O(N) algorithm where M is the number of subrepositories and N is the number of files in the batch.

The issue was most apparent when using VisualHG as this bug caused all files in repositories to be added to the main repository automatically
when opening solution (VisualHG automatically adds unknown files which are not ignored by .hgignore).