rebase: one safety net, one bugfix and one optimization.

When a .dotest from a previously failed rebase or patch
application exists, rebase got confused and tried to apply
mixture of what was already there and what is being rebased.
Check the existence of the directory and barf.

It failed with an mysterious "fatal: cannot read mbox" message
if the branch being rebased is fully in sync with the base.
Also if the branch is a proper descendant of the base, there is
no need to run rebase logic.  Prevent these from happening by
checking where the merge-base is.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-rebase.sh

@@ -5,9 +5,25 @@
 
 . git-sh-setup
 
-# The other head is given
+# Make sure we do not have .dotest
+if mkdir .dotest
+then
+	rmdir .dotest
+else
+	echo >&2 '
+It seems that I cannot create a .dotest directory, and I wonder if you
+are in the middle of patch application or another rebase.  If that is not
+the case, please rm -fr .dotest and run me again.  I am stopping in case
+you still have something valuable there.'
+	exit 1
+fi
+
+# The other head is given.  Make sure it is valid.
 other=$(git-rev-parse --verify "$1^0") || exit
 
+# Make sure we have HEAD that is valid.
+head=$(git-rev-parse --verify "HEAD^0") || exit
+
 # The tree must be really really clean.
 git-update-index --refresh || exit
 diff=$(git-diff-index --cached --name-status -r HEAD)
@@ -23,6 +39,16 @@ case "$#" in
 	git-checkout "$2" || exit
 esac
 
+# If the HEAD is a proper descendant of $other, we do not even need
+# to rebase.  Make sure we do not do needless rebase.  In such a
+# case, merge-base should be the same as "$other".
+mb=$(git-merge-base "$other" "$head")
+if test "$mb" = "$other"
+then
+	echo >&2 "Current branch `git-symbolic-ref HEAD` is up to date."
+	exit 0
+fi
+
 # Rewind the head to "$other"
 git-reset --hard "$other"
 git-format-patch -k --stdout --full-index "$other" ORIG_HEAD |