Avoid lockup when running without arguments and stdin is connected to a tty

[dotdash]

Up until commit 55287a8 "Run in diff mode iff two positional arguments
are supplied" diff mode was only supported when stdin is connected to a
tty, which meant that, for example, running delta a b in a git hook
would do the wrong thing.

As a fix, the tty check was removed, so now delta a b always works,
but the only error condition is now to pass only a single file. When
passing no files at all, we try parse to stdin even when connected to a
tty. Usually that wouldn't be that bad, because the user could press
Ctrl-C or Ctrl-D (EOF) to quit, unfortunately something in the pager
setup breaks this, and the only way to exit delta is to kill it.

Running delta with stdin connected to a tty would mean that the user
has to manually enter a diff (or paste it), which is not really an
expected scenario, as demonstrated by the error message that's currently
only emitted when running delta with only a single file argument. So
we can avoid the lockup by also emitting the error message when running
on a tty and there are no files to be diffed.

Fixes #1039, fixes #1090

[dandavison]

Thanks a lot, LGTM!