magit-wash-diffstat: match lines with no changes

Lines with no changes have only the count with 0.
magit-wash-diffstat: simplify regular expression
Merge pull request #1207 from afrepues/fix/truncated-diffstat-for-null-files

Fix/truncated diffstat for null files or no changes
Correct the documentation of `magit-push-dwim'.
magit-push-dwim: realign doc-string
Merge branch 'push-prefix'