AUTHORS.md: update list of contributors
magit-insert-stashes: work around issue in Git <1.7.10.3

Closes issue #1427.
magit-ediff: correct doc-string

The range is passed directly to `git diff' which, when given
a single commit, compares the working tree with that commit.
magit-commit-add-log: properly abort if nothing is committed

See #1438.