subrepo: (mostly) use relative path in "skipping missing subrepository"
This is consistent with the other messages printed by these
functions. Note that addremove is a little different and prints
absolute (aka repo-relative) paths if no argument was given.
Differential Revision: https://phab.mercurial-scm.org/D5900
$ hg init
$ echo a > a
$ hg ci -Am t
adding a
$ hg mv a b
$ hg ci -Am t1
$ hg debugrename b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3
$ hg mv b a
$ hg ci -Am t2
$ hg debugrename a
a renamed from b:37d9b5d994eab34eda9c16b195ace52c7b129980
$ hg debugrename --rev 1 b
b renamed from a:b789fdd96dc2f3bd229c1dd8eedf0fc60e2b68e3