checkout: quote invalid treeish in error message

checkout: quote invalid treeish in error message

We received a report that invoking "git restore -source my_base_branch"
resulted in the confusing error message "fatal: could not resolve
ource".  This looked like a typo in our error message, but it is
actually because "-source" is missing its second dash and is being
resolved as "-s ource".  However, due to the lack of the quoting
recommended in CodingGuidelines, this is confusing to the reader and
we can do better.

Add the necessary quoting to this message.  With this change, we now get
this less confusing message:

    fatal: could not resolve 'ource'

Reported-by: Zhelyo Zhelev <zhelyo@gmail.com>
Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>