We currently use `p_fnmatch` to compute whether a given "gitdir:"
or "gitdir/i:" conditional matches the current configuration file
path. As git.git has moved to use `wildmatch` instead of
`p_fnmatch` throughout its complete codebase, we evaluate
conditionals inconsistently with git.git in some special cases.

Convert `p_fnmatch` to use `wildmatch`. The `FNM_LEADINGDIR` flag
cannot be translated to `wildmatch`, but in fact git.git doesn't
use it here either. And in fact, dropping it while we go
increases compatibility with git.git.