filemerge: reduce some duplication in `_maketempfiles()`
The two callers of the local `maketempfrompath()` function used the
returned file object in the same way. We can reduce duplication by
moving that code into the function.
Differential Revision: https://phab.mercurial-scm.org/D12192
cat >> $HGRCPATH <<EOF
[extensions]
narrow=
[experimental]
changegroup3 = True
EOF