repos: prevent fs.move from failing

Prevent fs.move from failing with

```
File already exists: /usr/local/share/lua/5.1/luasec_0_6_2-ssl.lua
```

when overwriting a stale file. This should have already been handled,
but in case the manifest is out of sync for some reason this should
make the behavior more smooth.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/luarocks/repos.lua b/src/luarocks/repos.lua
index eaec87b8..0058d928 100644
--- a/src/luarocks/repos.lua
+++ b/src/luarocks/repos.lua
@@ -262,6 +262,7 @@ end
 
 local function backup_existing(should_backup, target)
    if not should_backup then
+      fs.delete(target)
       return
    end
    if fs.exists(target) then
@@ -301,6 +302,7 @@ local function op_rename(op)
 
    if fs.exists(op.src) then
       fs.make_dir(dir.dir_name(op.dst))
+      fs.delete(op.dst)
       local ok, err = fs.move(op.src, op.dst)
       fs.remove_dir_tree_if_empty(dir.dir_name(op.src))
       return ok, err