git-rebase -i: clean-up error check codepath.

After replaying a single change, the code performed a number of checks,
but some of them were for sanity checking, failures from which should
make the command abort, and others were checks to see if it should make
a new commit object.  Stringing them together with "&&" was wrong.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase--interactive.sh

@@ -365,17 +365,26 @@ do
 
 		test -d "$DOTEST" || die "No interactive rebase running"
 
-		# commit if necessary
-		git rev-parse --verify HEAD > /dev/null &&
-		git update-index --refresh &&
-		git diff-files --quiet &&
-		! git diff-index --cached --quiet HEAD -- &&
-		. "$DOTEST"/author-script && {
-			test ! -f "$DOTEST"/amend || git reset --soft HEAD^
-		} &&
-		export GIT_AUTHOR_NAME GIT_AUTHOR_EMAIL GIT_AUTHOR_DATE &&
-		if ! git commit --no-verify -F "$DOTEST"/message -e
+		# Sanity check
+		git rev-parse --verify HEAD >/dev/null ||
+			die "Cannot read HEAD"
+		git update-index --refresh && git diff-files --quiet ||
+			die "Working tree is dirty"
+
+		# do we have anything to commit?
+		if git diff-index --cached --quiet HEAD --
 		then
+			: Nothing to commit -- skip this
+		else
+			. "$DOTEST"/author-script ||
+				die "Cannot find the author identity"
+			if test -f "$DOTEST"/amend
+			then
+				git reset --soft HEAD^ ||
+				die "Cannot rewind the HEAD"
+			fi
+			export GIT_AUTHOR_NAME GIT_AUTHOR_EMAIL GIT_AUTHOR_DATE &&
+			git commit --no-verify -F "$DOTEST"/message -e ||
 			die "Could not commit staged changes."
 		fi