"filesystem validation" is vague.  These functions validate the length,
name them as such.

The repository-specific repository path validation belongs in the
repository class, not the generic path class.

examples: Free the git_config and git_config_entry after use

oidarray: introduce `git_oidarray_dispose`

buf: common_prefix takes a string array

`git_strarray` is a public-facing type.  Change
`git_buf_text_common_prefix` to not use it, and just take an array of
strings instead.