Fix plugin loses track of current state

[jofserponcon]

To fix the following scenario:

1- diff 2 directories
2- open a differ
3- Enter/o outside of the differ or only in range (e.g., on any of the 6 first lines)
4- open again the same differ line as in 2-

(Neovim 0.10.4 on MacOS)

=> The diff state is then broken:

• the ex command line height is now weirdly increased (I havent tracked why)
• it's now basically impossible to reenter a stable state, by opening a new differ or only in line

It seems to me that in the function DirDiffOpen(), we try too late to manage the case where we hit a non diff/only line.

After 3-:

• b:currentDiff refers to the new non-diff line
• s:FilenameA and s:FilenameB now contain the names of directories [A] and [B] respectively.

On 4-, when we enter DirDiffOpen() again, we don't exit the function right away because b:currentDiff does not match. Why not.

However, when we finally enter this block, the call to bufnr(directory A) returns the same buffer number as that of the actual file A in diff. So, we basically delete the buffer we just entered.

I think that the easiest is, during step 3, leave things as immaculate as possible:
like, if we see that we are not on a differ or only line, we just leave, which is what I propose in this PR.