Use nice names in conflict markers during cherry-pick/revert.

Always call the current HEAD 'HEAD', and name the patch being
cherry-picked or reverted by its oneline subject rather than
its SHA1.  This matches git am's behavior and is done because
users most commonly are cherry-picking by SHA1 rather than by
ref name.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-revert.sh

@@ -146,6 +146,12 @@ cherry-pick)
 
 esac >.msg
 
+eval GITHEAD_$head=HEAD
+eval GITHEAD_$next='`git show -s \
+	--pretty=oneline --encoding="$encoding" "$commit" |
+	sed -e "s/^[^ ]* //"`'
+export GITHEAD_$head GITHEAD_$next
+
 # This three way merge is an interesting one.  We are at
 # $head, and would want to apply the change between $commit
 # and $prev on top of us (when reverting), or the change between