iterator/diff: allow trailing `/` on start/end paths to match submodules

ignore: don't use realpath to canonicalize path

Allow callers to specify a start path with a trailing slash to match
a submodule, instead of just a directory.  This is for some legacy
behavior that's sort of dumb, but there it is.

Test that submodules are found when the are included in a pathspec
but have a trailing slash.

If we're looking for a symlink, realpath will give us the resolved path,
which is not what we're after, but a canonicalized version of the path
the user asked for.

merge_driver: fix missing `goto done;`

The code initializing the merge driver registry accidentally
forgot a `goto done` in case of an error. Because of this the
next line, which registers the global shutdown callback for the
merge drivers, is only called when an error occured.

Fix this by adding the missing `goto done`. This fixes some
memory leaks when the global state is shut down.

WD iterator: properly identify submodules

Plug some leaks

Add a no-op size_t typedef for the doc parser

Remove duplicated calls to git_mwindow_close

xprepare memleaks

Clang's documentation parser, which we use in our documentation system
does not report any comments for functions which use size_t as a type.

The root cause is buried somewhere in libclang but we can work around it
by defining the type ourselves. This typedef makes sure that libclang
sees it and that we do not change its size.

The xdl_prepare_env() function may initialise an xdlclassifier_t
data structure via xdl_init_classifier(), which allocates memory
to several fields, for example 'rchash', 'rcrecs' and 'ncha'.
If this function later exits due to the failure of xdl_optimize_ctxs(),
then this xdlclassifier_t structure, and the memory allocated to it,
is not cleaned up.

In order to fix the memory leak, insert a call to xdl_free_classifier()
before returning.

This patch was originally written by Ramsay Jones (see commit
87f16258367a3b9a62663b11f898a4a6f3c19d31 in git.git).

Commit 307ab20b3 ("xdiff: PATIENCE/HISTOGRAM are not independent option
bits", 19-02-2012) introduced the XDF_DIFF_ALG() macro to access the
flag bits used to represent the diff algorithm requested. In addition,
code which had used explicit manipulation of the flag bits was changed
to use the macros.

However, one example of direct manipulation remains. Update this code to
use the XDF_DIFF_ALG() macro.

This patch was originally written by Ramsay Jones (see commit
5cd6978a9cfef58de061a9525f3678ade479564d in git.git).

config: don't write duplicate section

config: don't special-case multivars that don't exist yet

If we hit the EOF while trying to write a new value, it may be that
we're already in the section that we were looking for.  If so, do not
write a (duplicate) section header, just write the value.

We should notice that we are in the correct section to add. This is a
cosmetic bug, since replacing any of these settings does work.

CMakeLists: Further improve the error messages regarding CMAKE_SIZEOF_VOID_P

Some FANTASTIC iterator refactoring

Remove some unused functions, refactor some ugliness.

(It's slow!)

Since the three iterators implement `advance_over` differently,
mandate it and implement each.