Remove MERGE_HEAD in "git checkout/reset"

Both of these scripts will end up resetting the index to some specific
head, and any unresolved merge will be forgotten.

git-reset-script

@@ -1,3 +1,5 @@
 #!/bin/sh
+: ${GIT_DIR=.git}
 git-read-tree --reset HEAD
 git-update-cache --refresh
+rm -f "$GIT_DIR/MERGE_HEAD"