linked worktrees unsupported when running build_docs

[drewdaemon]

This is a bit of an edge case, but documenting it here for the benefit of any future folks who may run into this issue.

If build_docs is pointed at an ascii-doc index file in a linked git worktree, it fails to identify the correct root directory for the git repo, leading to cascading failures.

STR

• clone Kibana
• from within the Kibana root directory, run git worktree add ../linked-kibana-worktree
• cd ../linked-kibana-worktree
• path/to/docs/repo/build_docs --doc ./docs/index.asciidoc

Result

INFO:build_docs:Building HTML from /doc/linked-kibana-worktree/docs/index.asciidoc
INFO:build_docs:Couldn't find repo toplevel for /doc/linked-kibana-worktree/docs
INFO:build_docs:Guessed toplevel=[/doc/linked-kibana-worktree/docs] remote=[0] branch=[master] repo=[repo]
...

Under the hood, the rev-parse command is failing with the following error (which is silenced in the current script).

INFO:build_docs:Error executing: git rev-parse --show-toplevel
INFO:build_docs:---out---
INFO:build_docs:
INFO:build_docs:---err---
INFO:build_docs:fatal: not a git repository: path/to/kibana/.git/worktrees/linked-kibana-worktree

This causes $toplevel to be empty and the script makes an incorrect guess and fails.