bash: use __gitdir when completing 'git rebase' options

When doing completion of rebase options in a subdirectory of the work
tree during an ongoing rebase, wrong options were offered because of the
hardcoded .git/.dotest-merge path.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

contrib/completion/git-completion.bash

@@ -841,8 +841,8 @@ _git_push ()
 
 _git_rebase ()
 {
-	local cur="${COMP_WORDS[COMP_CWORD]}"
-	if [ -d .dotest ] || [ -d .git/.dotest-merge ]; then
+	local cur="${COMP_WORDS[COMP_CWORD]}" dir="$(__gitdir)"
+	if [ -d .dotest ] || [ -d "$dir"/.dotest-merge ]; then
 		__gitcomp "--continue --skip --abort"
 		return
 	fi