fast-import.c: delete first, add/modify later

When doing a fast-import on a case-preserving filesystem, the fast-import
must ensure files are deleted before the other operations.  For example...

M 100644 :1 FileName.txt
D Filename.txt

...in a case sensitive filesystem will rename Filename.txt -> FileName.txt.
On a case insensitive or preserving  filesystem, however, both files (or
rather the one and only file) will be deleted.

To properly rename a file, delete the old file first, then recreate the
file with the new name.  IOW, process the above list in this order.

D Filename.txt
M 100644 :1 FileName.txt

Signed-off-by: Reuben Hawkins <reubenhwk@gmail.com>