tests/testlib/common.sh

convert: use the merge action constant

The constant exists, lets use them. Otherwise we cannot make these constant more
powerful.

Differential Revision: https://phab.mercurial-scm.org/D12113

mkcommit() {
   name="$1"
   shift
   echo "$name" > "$name"
   hg add "$name"
   hg ci -m "$name" "$@"
}