Install git pre-commit hook only if there's a directory to put it in

[umlaeute]

The assumption that we can put a file into .git/hooks/ if .git exists is wrong, e.g. if .git is a file (as is the case when using git submodule).

This PR fixes the issue by checking if .git/hooks/ exists instead:

• the trailing slash checks for directories (or: directory-likes) rather than for mere existence (regardless of type)
• since we want to put the target file into the hooks/ subdirectory we just check for that (just in case the .git directory is weird)

Closes: #603