Windows (Git Bash) is a nice guy

Features

• Warn (and ask confirmation) on prefix sharing across subtrees. e25cebc closes #17.

Fixes and tweaks

• Now works in Git Windows' Git Bash prompts (which are still using Bash 3.1) - 0545e1a closes #6 + 34ccff1.
• When on Windows, uses working basic glyphs for checkmarks and crosses so they read better - d91e9e7.
• Deprecation: git stree forget is now git stree clear. Less of a confusion risk with git stree rm. Also refuses to work when given arguments, to avoid unintentional clearing when a single-stree removal is intended - 6389511, also closes #9.
• git stree pull now properly defines the squashed commits log in the commit message, also allowing for customized log length, a la git merge --log. Large lists are no longer an issue, btw. 0c2879c closes #5.
• Stops relying on compatible push.default settings in git stree push. b096cc2 closes #13.