config.c: Make git_config() work correctly when called recursively

config.c: Make git_config() work correctly when called recursively

On Cygwin, this fixes a test failure in t3301-notes.sh (test 98,
"git notes copy --for-rewrite (disabled)").

The test failure is caused by a recursive call to git_config() which
has the effect of skipping to the end-of-file while processing the
"notes.rewriteref" config variable. Thus, any config variables that
appear after "notes.rewriteref" are simply ignored by git_config().
Also, we note that the original FILE handle is leaked as a result
of the recursive call.

The recursive call to git_config() is due to the "schizophrenic stat"
functions on cygwin, where one of two different implementations of
the l/stat functions is selected lazily, depending on some config
variables.

In this case, the init_copy_notes_for_rewrite() function calls
git_config() with the notes_rewrite_config() callback function.
This callback, while processing the "notes.rewriteref" variable,
in turn calls string_list_add_refs_by_glob() to process the
associated ref value. This eventually leads to a call to the
get_ref_dir() function, which in turn calls stat(). On cygwin,
the stat() macro leads to an indirect call to cygwin_stat_stub()
which, via init_stat(), then calls git_config() in order to
determine which l/stat implementation to bind to.

In order to solve this problem, we modify git_config() so that the
global state variables used by the config reading code is packaged
up and managed on a local state stack.

Signed-off-by: Ramsay Jones <ramsay@ramsay1.demon.co.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>