Fix tree refreshing logic

[yehy4]

This PR contains one commit to fix an issue in the logic that is used to prevent too many refreshes during a short period of time.

Before this commit, when lib.refresh_tree() is called with disable_clock = true (for example by typing R in the tree's window), refreshing gets set to true. However, nothing sets refreshing to false afterwards because the if statement evaluates to false. This results in subsequent calls to lib.refresh_tree() with disable_clock = false (for example through :NvimTreeRefresh) to exit the function early and therefore not actually update the tree.

The proposed fix in this commit is to simply set refreshing to false unconditionally (i.e., regardless of the disable_clock value).

P.S.: Thank you for you for creating and maintaining this great plugin!

[kyazdani42]

seen that indeed, was fixed in a big refacto i was doing, but still merging this in the meantime