help: explain in `hg help flags` that unambiguous prefixes are allowed
I used `hg commit --amend` as an example because that's the most
frequently used flag I could think of that doesn't yet have a short
form.
Differential Revision: https://phab.mercurial-scm.org/D8607
$ echo "[extensions]" >> $HGRCPATH
$ echo "mq=" >> $HGRCPATH
$ hg init
$ echo 'base' > base
$ hg ci -Ambase
adding base
$ hg qnew -mmqbase mqbase
$ hg qsave
$ hg qrestore 2
restoring status: hg patches saved state