filter-branch: stop special-casing $filter_subdir argument

Handling $filter_subdir in the usual way requires a separate case at
every use, because the variable is empty when unused.

Furthermore, --subdirectory-filter supplies its own '--', and if the user
provided one himself, such as in

  git filter-branch --subdirectory-filter subdir -- --all -- subdir/file

	an extra '--' was used as path filter in the call to git-rev-list that
determines the commits that shall be rewritten.

To keep the argument handling sane, we filter $@ to contain only the
non-revision arguments, and store all revisions in $ref_args.  The
$ref_args are easy to handle since only the SHA1s are needed; the
actual branch names have already been stored in $tempdir/heads at this
point.

An extra separating -- is only required if the user did not provide
any non-revision arguments, as the latter disambiguate the
$filter_subdir following after them (or fail earlier because they are
ambiguous themselves).

Thanks to Johannes Sixt for suggesting this solution.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-filter-branch.sh

@@ -257,15 +257,24 @@ git read-tree || die "Could not seed the index"
 # map old->new commit ids for rewriting parents
 mkdir ../map || die "Could not create map/ directory"
 
+# we need "--" only if there are no path arguments in $@
+nonrevs=$(git rev-parse --no-revs "$@") || exit
+test -z "$nonrevs" && dashdash=-- || dashdash=
+rev_args=$(git rev-parse --revs-only "$@")
+
 case "$filter_subdir" in
 "")
-	git rev-list --reverse --topo-order --default HEAD \
+	eval set -- "$(git rev-parse --sq --no-revs "$@")"
-		--parents --simplify-merges "$@"
 	;;
 *)
-	git rev-list --reverse --topo-order --default HEAD \
+	eval set -- "$(git rev-parse --sq --no-revs "$@" $dashdash \
-		--parents --simplify-merges "$@" -- "$filter_subdir"
+		"$filter_subdir")"
-esac > ../revs || die "Could not get the commits"
+	;;
+esac
+
+git rev-list --reverse --topo-order --default HEAD \
+	--parents --simplify-merges $rev_args "$@" > ../revs ||
+	die "Could not get the commits"
 commits=$(wc -l <../revs | tr -d " ")
 
 test $commits -eq 0 && die "Found nothing to rewrite"
@@ -356,8 +365,7 @@ then
 	do
 		sha1=$(git rev-parse "$ref"^0)
 		test -f "$workdir"/../map/$sha1 && continue
-		ancestor=$(git rev-list --simplify-merges -1 \
+		ancestor=$(git rev-list --simplify-merges -1 "$ref" "$@")
-				$ref -- "$filter_subdir")
 		test "$ancestor" && echo $(map $ancestor) >> "$workdir"/../map/$sha1
 	done < "$tempdir"/heads
 fi