Keep config comments in the same place as git

When there is a comment at the end of a section, git keeps it there,
while we write the new variable right at the end.

Keep comments buffered and dump them when we're going to output a
variable or section, or reach EOF. This puts us in line with the config
files which git produces.

pkg-config: fix directory references in libgit2.pc

Before:
libdir=/usr//usr/lib64
includedir=/usr//usr/include

After:
libdir=/usr/lib64
includedir=/usr/include

(note the duplication of /usr in the before case)

`mkdir`: split into `mkdir` and `mkdir_relative`

Don't coalesce all errors into ENOENT.  At least identify EACCES.
All callers should be handling this case already, as the POSIX
`lstat` will return this.

`git_futils_mkdir` does not blindly call `git_futils_mkdir_relative`.

`git_futils_mkdir_relative` is used when you have some base directory
and want to create some path inside of it, potentially removing blocking
symlinks and files in the process.  This is not suitable for a general
recursive mkdir within the filesystem.

Instead, when `mkdir` is being recursive, locate the first existent
parent directory and use that as the base for `mkdir_relative`.

In `mkdir` and `mkdir_r`, ensure that we don't try to remove symlinks
that are in our way.

Untangle git_futils_mkdir from git_futils_mkdir_ext - the latter
assumes that we own everything beneath the base, as if it were
being called with a base of the repository or working directory,
and is tailored towards checkout and ensuring that there is no
bogosity beneath the base that must be cleaned up.

This is (at best) slow and (at worst) unsafe in the larger context
of a filesystem where we do not own things and cannot do things like
unlink symlinks that are in our way.

Checkout: handle mode changes

When a file exists on disk and we're checking out a file that differs
in executableness, remove the old file.  This allows us to recreate the
new file with p_open, which will take the new mode into account and
handle setting the umask properly.

Remove any notion of chmod'ing existing files, since it is now handled
by the aforementioned removal and was incorrect, as it did not take
umask into account.

Clone test buffer

Handle `git_path_diriter` instances at the drive root on Windows

iterator: loop fs_iterator advance (don't recurse)

The canonical directory path of the root directory of a volume on
POSIX already ends in a slash (eg, `/`).  This is true only at the
root.  Do not add a slash to paths in this case.

The canonical directory path of the root directory of a volume on
windows already ends in a slash (eg, `c:/`).  This is true only
at the volume root.  Do not add a slash to paths in this case.

Ensure that we can iterate the filesystem root and that paths come
back well-formed, not with an additional '/'.  (eg, when iterating
`c:/`, expect that we do not get some path like `c://autoexec.bat`).

push: put the git_oid inline in the test structure

iterator: advance the tree iterator smartly

Don't free config in `git_transaction_commit`.

The config is not owned by the transaction, so please don’t free it.

The previous commit left the comment referencing the earlier state of
the code, change it to explain the current logic. While here, change the
logic to avoid repeating the copy of the base pattern.

submodule: refactor to be more explicit in the search

These are small pieces of data, so there is no advantage to allocating
them separately. Include the two ids inline in the struct we use to
check that the expected and actual ids match.