config_file: split out module to parse config files

The configuration file code grew quite big and intermingles both actual
configuration logic as well as the parsing logic of the configuration
syntax. This makes it hard to refactor the parsing logic on its own and
convert it to make use of our new parsing context module.

Refactor the code and split it up into two parts. The config file code
will only handle actual handling of configuration files, includes and
writing new files. The newly created config parser module is then only
responsible for parsing the actual contents of a configuration file,
leaving everything else to callbacks provided to its provided function
`git_config_parse`.

src/config_parse.h

+/*
+ * Copyright (C) the libgit2 contributors. All rights reserved.
+ *
+ * This file is part of libgit2, distributed under the GNU GPL v2 with
+ * a Linking Exception. For full terms see the included COPYING file.
+ */
+
+#include "common.h"
+#include "array.h"
+#include "oid.h"
+
+static const char *git_config_escapes = "ntb\"\\";
+static const char *git_config_escaped = "\n\t\b\"\\";
+
+typedef struct config_file {
+	git_oid checksum;
+	char *path;
+	git_array_t(struct config_file) includes;
+} git_config_file;
+
+typedef struct {
+	struct config_file *file;
+	git_buf buffer;
+	char *read_ptr;
+	int line_number;
+	int eof;
+} git_config_parser;
+
+typedef int (*git_config_parser_section_cb)(
+	git_config_parser *parser,
+	const char *current_section,
+	const char *line,
+	size_t line_len,
+	void *data);
+
+typedef int (*git_config_parser_variable_cb)(
+	git_config_parser *parser,
+	const char *current_section,
+	char *var_name,
+	char *var_value,
+	const char *line,
+	size_t line_len,
+	void *data);
+
+typedef int (*git_config_parser_comment_cb)(
+	git_config_parser *parser,
+	const char *line,
+	size_t line_len,
+	void *data);
+
+typedef int (*git_config_parser_eof_cb)(
+	git_config_parser *parser,
+	const char *current_section,
+	void *data);
+
+int git_config_parse(
+	git_config_parser *parser,
+	git_config_parser_section_cb on_section,
+	git_config_parser_variable_cb on_variable,
+	git_config_parser_comment_cb on_comment,
+	git_config_parser_eof_cb on_eof,
+	void *data);