mv: name both source and destination when rename fails

mv: name both source and destination when rename fails

When "git mv" fails at the rename(2) syscall, the error is reported
with die_errno() using only the source path:

    fatal: renaming 'src' failed: No such file or directory

rename(2) returns ENOENT both when the source does not exist and when
a directory component of the destination does not exist, and errno
does not distinguish the two. Reporting only the source therefore
misleads the user in the latter case: for

    git mv a/file b/no-such-dir/file

the message blames 'a/file', which exists, and gives no hint that
'b/no-such-dir/' is the missing part.

Inspecting the paths again after the failure to determine which one is
at fault would be racy, since either could appear or disappear between
the rename(2) and the follow-up check. Instead, simply name both the
source and the destination in the message and let the reader see which
one is wrong:

    fatal: renaming 'a/file' to 'b/no-such-dir/file' failed:
    No such file or directory

Signed-off-by: Lucas Zamboni Orioli <lucaszam0@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>