tests: flatten repo structure in test-commit.t

Let's move to parent directory before `hg init` repos, since they don't need to
be nested. It makes amend/strip messages that include full path to the backup
bundle shorter, for instance.

tests/test-commit.t

@@ -708,6 +708,8 b' verify pathauditor blocks evil filepaths'
   abort: path contains illegal component: HG8B6C~2/hgrc
   [255]
 
+  $ cd ..
+
 # test that an unmodified commit template message aborts
 
   $ hg init unmodified_commit_template
@@ -734,6 +736,8 b' verify pathauditor blocks evil filepaths'
   abort: commit message unchanged
   [255]
 
+  $ cd ..
+
 test that text below the --- >8 --- special string is ignored
 
   $ cat <<'EOF' > $TESTTMP/lowercaseline.sh