diffhelpers: backport
9e40bc4c1bde from C implementation
9e40bc4c1bde just says "harden testhunk." I don't think this would be
the case, but it makes some sense to avoid negative index.
Failed qimport of patches from files should cleanup by recording successfully
imported patches in series file.
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am'add a'
adding a
$ cat >b.patch<<EOF
> diff --git a/a b/a
> --- a/a
> +++ b/a
> @@ -1,1 +1,2 @@
> a
> +b
> EOF
empty series
$ hg qseries
qimport valid patch followed by invalid patch
$ hg qimport b.patch fakepatch
adding b.patch to series file
abort: unable to read file fakepatch
[255]
valid patches before fail added to series
$ hg qseries
b.patch
$ hg pull -q -r 0 . # update phase
$ hg qimport -r 0
abort: revision 0 is not mutable
(see 'hg help phases' for details)
[255]
$ cd ..