+
+# The tree must be really really clean.
+git-update-index --refresh || exit
+diff=$(git-diff-index --cached --name-status -r HEAD)
+case "$diff" in
+?*) echo "$diff"
+ exit 1
+ ;;
+esac
+
+# The upstream head must be given. Make sure it is valid.
+upstream_name="$1"
+upstream=`git rev-parse --verify "${upstream_name}^0"` ||
+ die "invalid upstream $upstream_name"
+
+# If a hook exists, give it a chance to interrupt
+if test -x "$GIT_DIR/hooks/pre-rebase"
+then
+ "$GIT_DIR/hooks/pre-rebase" ${1+"$@"} || {
+ echo >&2 "The pre-rebase hook refused to rebase."
+ exit 1
+ }
+fi
+
+# If the branch to rebase is given, first switch to it.
+case "$#" in
+2)
+ branch_name="$2"
+ git-checkout "$2" || usage
+ ;;
+*)
+ branch_name=`git symbolic-ref HEAD` || die "No current branch"
+ branch_name=`expr "$branch_name" : 'refs/heads/\(.*\)'`
+ ;;
+esac
+branch=$(git-rev-parse --verify "${branch_name}^0") || exit
+
+# Make sure the branch to rebase onto is valid.
+onto_name=${newbase-"$upstream_name"}
+onto=$(git-rev-parse --verify "${onto_name}^0") || exit
+
+# Now we are rebasing commits $upstream..$branch on top of $onto
+
+# Check if we are already based on $onto, but this should be
+# done only when upstream and onto are the same.
+if test "$upstream" = "onto"
+then
+ mb=$(git-merge-base "$onto" "$branch")
+ if test "$mb" = "$onto"
+ then
+ echo >&2 "Current branch $branch_name is up to date."
+ exit 0
+ fi
+fi
+
+# Rewind the head to "$onto"; this saves our current head in ORIG_HEAD.
+git-reset --hard "$onto"
+
+# If the $onto is a proper descendant of the tip of the branch, then
+# we just fast forwarded.
+if test "$mb" = "$onto"
+then
+ echo >&2 "Fast-forwarded $branch to $newbase."
+ exit 0
+fi
+
+git-format-patch -k --stdout --full-index "$upstream" ORIG_HEAD |
+git am --binary -3 -k