version: relax git detection logic (again)

This is a repeat of commit 3aa68cd397
which was lost in a rework of version.sh.

git worktrees have a .git file rather than a .git directory, so building
in a worktree caused version.sh to generate an error.

Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>

version/version.sh

@@ -31,7 +31,7 @@ case $# in
         if [ -z "$extra_hash_or_dir" ]; then
             # Nothing, empty extra hash is fine.
             extra_hash=""
-        elif [ -d "$extra_hash_or_dir/.git" ]; then
+        elif [ -e "$extra_hash_or_dir/.git" ]; then
             extra_hash=$(git_hash_dirty "$extra_hash_or_dir" HEAD)
         elif ! expr "$extra_hash_or_dir" : "^[0-9a-f]*$"; then
             echo "Invalid extra hash '$extra_hash_or_dir', must be a git commit or path to a git repo" >&2