tests/narrow-library.sh

commitctx: extract the function that commit a new manifest

The logic is large enough and isolated enough to be extracted, this reduce the
size of the main function, making it simpler to follow.

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