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()".
Test issue2761
$ hg init
$ touch to-be-deleted
$ hg add
adding to-be-deleted
$ hg ci -m first
$ echo a > to-be-deleted
$ hg ci -m second
$ rm to-be-deleted
$ hg diff -r 0
Same issue, different code path
$ hg up -C
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ touch does-not-exist-in-1
$ hg add
adding does-not-exist-in-1
$ hg ci -m third
$ rm does-not-exist-in-1
$ hg diff -r 1