Fixed git-subrepo and tests if set ff=only

Some tests failed, when ff_only is set in users git configuration;
like:
...
test/branch-rev-list-one-path.t .... Died at line 23 in main of test/branch-rev-list-one-path.t
test/branch-rev-list-one-path.t .... No subtests run
test/branch-rev-list.t ............. Died at line 26 in main of test/branch-rev-list.t
test/branch-rev-list.t ............. No subtests run
...

and git configuration is set like:
    $ git config --global pull.ff only
    $ git config --global merge.ff only

lib/git-subrepo

@@ -574,7 +574,7 @@ subrepo:pull() {
     fi
   else
     o "Merge in changes from $refs_subrepo_fetch"
-    FAIL=false RUN git merge "$refs_subrepo_fetch"
+    FAIL=false RUN git merge --ff "$refs_subrepo_fetch"
     if ! OK; then
       say "The \"git merge\" command failed:"
       say

test/branch-rev-list-one-path.t

@@ -19,7 +19,7 @@ subrepo-clone-bar-into-foo
   add-new-files bar/file3
   add-new-files bar/file4
   add-new-files bar/file5
-  git merge master
+  git merge --ff master
 ) >& /dev/null || die
 
 test-exists "$OWNER/foo/bar/file1" "$OWNER/foo/bar/file2" "$OWNER/foo/bar/file3" "$OWNER/foo/bar/file4" "$OWNER/foo/bar/file5"

test/branch-rev-list.t

@@ -22,7 +22,7 @@ subrepo-clone-bar-into-foo
   add-new-files bar/file3
   add-new-files bar/file4
   add-new-files bar/file5
-  git merge master
+  git merge --ff master
 ) >& /dev/null || die
 
 test-exists "$OWNER/foo/bar/file1" "$OWNER/foo/bar/file2" "$OWNER/foo/bar/file3" "$OWNER/foo/bar/file4" "$OWNER/foo/bar/file5"