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
https://bz.mercurial-scm.org/1089
$ hg init
$ mkdir a
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ hg ci -m m a
$ mkdir a b
$ echo a > a/b
$ hg ci -Am m
adding a/b
$ hg rm a
removing a/b
$ cd b
Relative delete:
$ hg ci -m m ../a
$ cd ..