spacemacs: Set undo-tree history directory to avoid polluting working directories
spacemacs: Set undo-tree history directory to avoid polluting working directories

A long time ago, spacemacs explicitly enabled undo-tree history auto-save by
setting `undo-tree-auto-save-history' to t and configuring
`undo-tree-history-directory-alist', but these settings were later removed in an
attempt to fix an issue:
https://github.com/syl20bnr/spacemacs/commit/885d092e72aeaa470253c19831ba42e2eecf3514#diff-cf66f7b82b078ca6858049c89d6a2c73f94084135d822c2f00f511b3cbbfb835L1885-R1888

At the time, removing these settings disabled history auto-save because the
default value of `undo-tree-auto-save-history' was nil. However, a recent
undo-tree update (version 0.8) changed the default to t:
https://gitlab.com/tsc25/undo-tree/-/commit/5da2a7aee98393d26a93c499dc79fcf793f161e1#0a9df8940aca5f4f6f6eabbd765d0c49f21a9ac9_942_943

This means that history auto-save is now implicitly enabled, and by default
history files are placed next to edited files, thus polluting working
directories. By setting `undo-tree-history-directory-alist' like spacemacs used
to, history files can instead be saved to a dedicated directory.

Hopefully, history auto-save is no longer problematic. If it still is, I'll just
disable it.

razzolini/dotfiles,razzolini/dotfiles,razzolini/dotfiles