Improve error output of git-rebase

"git rebase" without arguments on initial startup showed:

	fatal: Needed a single revision
	invalid upstream

This patch makes it show the ordinary usage string.

If .git/rebase-merge or .git/rebase-apply/rebasing exists, git-rebase
will die with a message saying that a rebase is in progress and the user
should try --skip/--abort/--continue.

If .git/rebase-apply/applying exists, git-rebase will die with a message
saying that git-am is in progress, regardless how many arguments are
given.

If no arguments are given and .git/rebase-apply/ exists, but neither a
rebasing nor applying file is in that directory, git-rebase dies with a
message saying that rebase-apply exists and no arguments were given.

Signed-off-by: Stephan Beyer <s-beyer@gmx.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-rebase.sh

@@ -144,8 +144,19 @@ is_interactive () {
 	done && test -n "$1"
 }
 
+test -f "$GIT_DIR"/rebase-apply/applying &&
+	die 'It looks like git-am is in progress. Cannot rebase.'
+
 is_interactive "$@" && exec git-rebase--interactive "$@"
 
+if test $# -eq 0
+then
+	test -d "$dotest" -o -d "$GIT_DIR"/rebase-apply || usage
+	test -d "$dotest" -o -f "$GIT_DIR"/rebase-apply/rebasing &&
+		die 'A rebase is in progress, try --continue, --skip or --abort.'
+	die "No arguments given and $GIT_DIR/rebase-apply already exists."
+fi
+
 while test $# != 0
 do
 	case "$1" in
@@ -268,16 +279,16 @@ done
 # Make sure we do not have $GIT_DIR/rebase-apply
 if test -z "$do_merge"
 then
-	if mkdir "$GIT_DIR"/rebase-apply
+	if mkdir "$GIT_DIR"/rebase-apply 2>/dev/null
 	then
 		rmdir "$GIT_DIR"/rebase-apply
 	else
 		echo >&2 '
-It seems that I cannot create a '"$GIT_DIR"'/rebase-apply directory,
-and I wonder if you are in the middle of patch application or another
+It seems that I cannot create a rebase-apply 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 '"$GIT_DIR"'/rebase-apply
- and run me again.  I am stopping in case you still have something
+and run me again.  I am stopping in case you still have something
 valuable there.'
 		exit 1
 	fi
@@ -285,7 +296,7 @@ else
 	if test -d "$dotest"
 	then
 		die "previous rebase directory $dotest still exists." \
-			'try git-rebase < --continue | --abort >'
+			'Try git rebase (--continue | --abort | --skip)'
 	fi
 fi