`magit-revert-item': don't depend on `cl-flet'

Commit 1c7fb237 replaced the `magit-with-revert-confirmation' macro with
the temporary function `confirm', but defined the latter using `cl-flet'
which isn't available in cl-lib 0.3, so use `let' + `funcall' instead.

Fixes #895.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-revert-item': respect `magit-revert-item-confirm'

Commit 1c7fb237 replaced the `magit-with-revert-confirmation' macro with
the temporary function `confirm', but in doing so also removed the check
for `magit-revert-item-confirm', causing `magit-revert-item' to always
ask for confirmation, even if the user configured it not to.

Fixes #896.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-diff-with-mark': better prompt and completion

When prompting for a rev to diff the currently marked commit with,
report the latter, and default to the currently checked out branch
unless that *is* the marked commit.

Signed-off-by: Pieter Praet <pieter@praet.org>
`magit-remote-ref-format': cleanup docstring

- Reword the first line, and fix a typo ("pariticular" -> "particular").
- Single-quote symbols.
- Enumerate the possible values more concisely.

Signed-off-by: Pieter Praet <pieter@praet.org>
Merge pull request #914 from praet/diff-with-mark-v2

diff-with-mark v2
Merge pull request #915 from praet/remote-ref-format-doc

`magit-remote-ref-format': cleanup docstring
Merge pull request #897 from praet/revert

Revert