Fix: Allow git worktree in version detection; drop svn, hg.

[kernigh]

If $ROOT_DIR is a linked working tree from git-worktree(1), then
$ROOT_DIR/.git is a regular file instead of a directory. Allow this
when deciding whether to use git to detect OpenTTD's version.

Drop checks for svn and hg in config.lib, because findversion.sh
hasn't used svn nor hg since 192770e.

[kernigh]

(Also, git commit from a linked tree fails, because the openttd_hooks can't find the hooks directory. I used HOOKS_DIR=$(git rev-parse --git-path hooks) git commit to make this commit.)

[nielsmh]

Looks fine at first glance, might have to be redone for #7270 (CMake) at some point.

[LordAro]

Seems to me that both -f and -d checks are unnecessary? Just -e should do

[kernigh]

Yes, -e also works. I put both -d and -f to show that .git can be a regular file. If I put -e, I fear that someone would later change the -e back to -d.

I see that #7270 CMake uses IS_DIRECTORY "${CMAKE_SOURCE_DIR}/.git". I would propose to change IS_DIRECTORY to EXISTS (because CMake has no IS_FILE), and add a comment that .git may be a directory or a regular file. I like the CMake script better than our shell scripts, because the check for .git is in 1 place, not 2.

[LordAro]

Fair enough, it'll do for now, especially if it's about to be rewritten Soon(tm)