rust-dirs-multiset: improve temporary error message
While we wait on a future patch that could verify that the paths passed to
`DirsMultiset` have been audited, we still need to handle this error.
This patch makes it easier to bubble up and makes the error clearer.
Also, this patch introduces the `subslice_index` function that could be useful
for other - albeit niche - purposes.
Differential Revision: https://phab.mercurial-scm.org/D7921
Test for changeset ba7c74081861
(update dirstate correctly for non-branchmerge updates)
$ hg init a
$ cd a
$ echo a > a
$ hg add a
$ hg commit -m a
$ cd ..
$ hg clone a b
updating to branch default
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ cd a
$ hg mv a b
$ hg commit -m move
$ echo b >> b
$ hg commit -m b
$ cd ../b
$ hg pull ../a
pulling from ../a
searching for changes
adding changesets
adding manifests
adding file changes
added 2 changesets with 2 changes to 1 files
new changesets e3c9b40284e1:772b37f1ca37
(run 'hg update' to get a working copy)
$ hg update
1 files updated, 0 files merged, 1 files removed, 0 files unresolved
$ cd ..