Remove duplicated calls to git_mwindow_close

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.

Some FANTASTIC iterator refactoring