subrepo: use relative path for "already tracked" message
From 932de135041f (subrepo: warn when adding already tracked files in
gitsubrepo, 2015-02-27):
The file is printed with abs() to be consistent with how it is
printed in workingctx, even though that is inconsistent with how
added files are printed in verbose mode.
However, a few year later, the same author wrote 7008f6819002
(context: name files relative to cwd in warning messages, 2017-07-11)
and now it's inconsistent. This fixes that inconsistency.
Differential Revision: https://phab.mercurial-scm.org/D5899
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