magit-reflog-enable: new function
magit.el: absorb magit-wip.el
rename magit-wip-{echo-area => save}-message
magit-wip-save-message: improve doc-string
magit-wip-{commit,save}-message: change format characters
magit-wip-commit-message: change default value
magit-wip-save-mode-lighter: make customizable

Also append a "M" (for Magit) to the default value.
turn-on-magit-wip-save: improve doc-string
teach magit-wip-save-mode to not depend on git-wip script

Rewrite `magit-wip-save' to do all the work without
requiring the (unmaintained) third-party script `git-wip'
from https://github.com/bartman/git-wip.  These two implementations
are compatible, provided the new option `magit-wip-ref-format' is
not customized.
Merge branch 'n/standalone-wip' into next