backout: avoid redundant message examination
Before this patch, "hg backout" examines "--message" and "--logfile"
options explicitly.
But this examination is redundant, because "commitfunc()" can receive
the result of same examination by "cmdutil.logmessage()" in
"cmdutil.commit()" through "message" argument.
This patch avoids redundant message examination by "message"
examination in "commitfunc()".
Testing that hghave does not crash when checking features
$ "$TESTDIR/hghave" --test-features 2>/dev/null