magit-log-file: don't use --follow

It corrupts the graph, even for files that have never been renamed.
magit-log-file: ignore prefix argument

Respect `magit-log-arguments' and do not add or remove `--graph'
depending on whether a prefix argument was used or not.
magit-log-file: always show visited file, remove from popup

Remove `magit-log-file' from the `magit-log-popup' and stop reading
a file from the user and instead unconditionally show the log for
the visited file.  That's its only purpose now, to make that more
obvious rename it to `magit-log-buffer-file'.

The log commands that remain in the log popup will soon be taught how
to limit the history to a file or multiple files, while keeping their
other features which `magit-log-file' lacked.
magit-log{,-current,-head}: allow limiting to files

Teach log commands that are commonly invoked from the log popup to
limit history to commits which touch certain files selected in the
popup using the new "-- " argument.

Add `magit-log-read-files', which uses `completing-read-multiple', to
read multiple files from the user.  Store the read value as a single
string, because that's all that the current popup implementation
supports.  Then in `magit-log-read-args' separate that string from
the other arguments, split it into a list of files again, and return
that list separate from the other arguments.

Adjust `magit-log-refresh-buffer', `magit-insert-log` and
`magit-insert-log-verbose' accordingly.

Remove `--relative' from `magit-log-popup', it's no longer required and
confusing to non-experts.
Merge branch 'n/log-files' into next