fast-import: allow unquoted empty path for root

fast-import: allow unquoted empty path for root

Ever since filerename was added in f39a946a1f (Support wholesale
directory renames in fast-import, 2007-07-09) and filecopy in b6f3481bb4
(Teach fast-import to recursively copy files/directories, 2007-07-15),
both have produced an error when the destination path is empty. Later,
when support for targeting the root directory with an empty string was
added in 2794ad5244 (fast-import: Allow filemodify to set the root,
2010-10-10), this had the effect of allowing the quoted empty string
(`""`), but forbidding its unquoted variant (``). This seems to have
been intended as simple data validation for parsing two paths, rather
than a syntax restriction, because it was not extended to the other
operations.

All other occurrences of paths (in filemodify, filedelete, the source of
filecopy and filerename, and ls) allow both.

For most of this feature's lifetime, the documentation has not
prescribed the use of quoted empty strings. In e5959106d6
(Documentation/fast-import: put explanation of M 040000 <dataref> "" in
context, 2011-01-15), its documentation was changed from “`<path>` may
also be an empty string (`""`) to specify the root of the tree” to “The
root of the tree can be represented by an empty string as `<path>`”.

Thus, we should assume that some front-ends have depended on this
behavior.

Remove this restriction for the destination paths of filecopy and
filerename and change tests targeting the root to test `""` and ``.

Signed-off-by: Thalia Archibald <thalia@archibald.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>