gitcmds: move more functions to gitcmds

Rename all_files() to tracked_files().

Add a new all_files() that actually returns all files and adjust callers
to use the one in gitcmds rather than via the model.

Use tracked_files() in the completion widgets since `git log` does not
care about untracked or newly-added files, so there's no sense in them
showing up in the completion.

Signed-off-by: David Aguilar <davvid@gmail.com>
widgets.completion: do not offer completions when text has no matches

The "show all refs when no matches are found" makes the filename matches
found by the GitLogCompletionModel hard to see since they are buried
below the list of refs.

Remove this feature so that the actual matches bubble to the top rather
than being obscured by a potentially huge list of refs.

Signed-off-by: David Aguilar <davvid@gmail.com>