Allow rebase to run if upstream is completely merged

Consider this history:

  o--o-...-B          <- origin
      \     \
       x--x--M--x--x  <- master

In this situation, rebase considers master fully up-to-date and would
not do anything. However, if there were additional commits on origin,
the rebase would run and move the commits x on top of origin.

Here we change rebase to short-circuit out only if the history since origin
is strictly linear. Consequently, the above as well as a history like this
would be linearized:

  o--o               <- origin
      \
       x--x
        \  \
         x--M--x--x  <- master

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 5 insertions, 3 deletions

diff --git a/git-rebase.sh b/git-rebase.sh
index c590661179..7a02f2975d 100755
--- a/git-rebase.sh
+++ b/git-rebase.sh
@@ -305,10 +305,12 @@ branch=$(git rev-parse --verify "${branch_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.
+# Check if we are already based on $onto with linear history,
+# but this should be done only when upstream and onto are the same.
 mb=$(git merge-base "$onto" "$branch")
-if test "$upstream" = "$onto" && test "$mb" = "$onto"
+if test "$upstream" = "$onto" && test "$mb" = "$onto" &&
+	# linear history?
+	! git rev-list --parents "$onto".."$branch" | grep " .* " > /dev/null
 then
 	echo >&2 "Current branch $branch_name is up to date."
 	exit 0