tests/narrow-library.sh

extdiff: avoid double backslashes in the displayed tool path on Windows

This shows the tool path in the help, and changed in 67b180c0e263.  uirepr()
already does the same thing, but that undoes the mangling in its call to repr().

cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[ui]
ssh=python "$TESTDIR/dummyssh"
[experimental]
changegroup3 = True
EOF