Windows: use %PROGRAMDATA%\Git\config as Windows-wide configuration

Between the libgit2 and the Git for Windows project, there has been a
discussion how we could share Git configuration to avoid duplication (or
worse: skew).

Earlier, libgit2 was nice enough to just re-use Git for Windows'

	C:\Program Files (x86)\Git\etc\gitconfig

but with the upcoming Git for Windows 2.x, there would be more paths to
search, as we will have 64-bit and 32-bit versions, and the
corresponding config files will be in %PROGRAMFILES%\Git\mingw64\etc and
...\mingw32\etc, respectively.

Therefore we came to a consensus to use %PROGRAMDATA%\Git as the
location for Git-specific files that are of wider interest than just Git
for Windows.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

compat/mingw.c

@@ -2128,3 +2128,12 @@ void mingw_startup()
 	/* initialize Unicode console */
 	winansi_init();
 }
+
+const char *windows_wide_config(void)
+{
+	static struct strbuf windows_wide = STRBUF_INIT;
+	if (!windows_wide.len)
+		strbuf_addf(&windows_wide,
+			"%s\\Git\\config", getenv("PROGRAMDATA"));
+	return windows_wide.buf;
+}

compat/mingw.h

@@ -359,6 +359,8 @@ static inline char *mingw_find_last_dir_sep(const char *path)
 int mingw_offset_1st_component(const char *path);
 #define offset_1st_component mingw_offset_1st_component
 #define PATH_SEP ';'
+extern const char *windows_wide_config(void);
+#define git_super_config windows_wide_config
 #define PRIuMAX "I64u"
 #define PRId64 "I64d"
 

config.c

@@ -1185,11 +1185,18 @@ int git_config_system(void)
 int git_config_early(config_fn_t fn, void *data, const char *repo_config)
 {
 	int ret = 0, found = 0;
+	const char *super_config = git_super_config();
 	char *xdg_config = NULL;
 	char *user_config = NULL;
 
 	home_config_paths(&user_config, &xdg_config, "config");
 
+	if (super_config && git_config_system() &&
+			!access(super_config, R_OK)) {
+		ret += git_config_from_file(fn, super_config, data);
+		found += 1;
+	}
+
 	if (git_config_system() && !access_or_die(git_etc_gitconfig(), R_OK, 0)) {
 		ret += git_config_from_file(fn, git_etc_gitconfig(),
 					    data);

git-compat-util.h

@@ -301,6 +301,10 @@ static inline char *git_find_last_dir_sep(const char *path)
 #define find_last_dir_sep git_find_last_dir_sep
 #endif
 
+#ifndef git_super_gitconfig
+#define git_super_gitconfig NULL
+#endif
+
 #if defined(__HP_cc) && (__HP_cc >= 61000)
 #define NORETURN __attribute__((noreturn))
 #define NORETURN_PTR