last-modified: rewrite error message when more than one commit given

last-modified: rewrite error message when more than one commit given

When more than one commit is passed to the git-last-modified(1) command,
this error message was printed:

    error: last-modified can only operate on one tree at a time

Calling these a "tree" is technically not correct. git-last-modified(1)
expects revisions that peel to a commit.

Rephrase the error message to:

    error: last-modified can only operate on one commit at a time

While at it, modify the test to ensure the correct error message is
printed.

Signed-off-by: Toon Claes <toon@iotcl.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>