errors: introduce git_error_vset function

Right now, we only provide a `git_error_set` that has a variadic
function signature. It's impossible to drive this function in a
C89-compliant way from other functions that have a variadic
signature, though, like for example `git_parse_error`.

Implement a new `git_error_vset` function that gets a `va_list`
as parameter, fixing the above problem.

Author: pks-t

src/errors.c

@@ -49,21 +49,26 @@ void git_error_set_oom(void)
 	GIT_GLOBAL->last_error = &g_git_oom_error;
 }
 
-void git_error_set(int error_class, const char *string, ...)
+void git_error_set(int error_class, const char *fmt, ...)
+{
+	va_list ap;
+
+	va_start(ap, fmt);
+	git_error_vset(error_class, fmt, ap);
+	va_end(ap);
+}
+
+void git_error_vset(int error_class, const char *fmt, va_list ap)
 {
-	va_list arglist;
 #ifdef GIT_WIN32
 	DWORD win32_error_code = (error_class == GIT_ERROR_OS) ? GetLastError() : 0;
 #endif
 	int error_code = (error_class == GIT_ERROR_OS) ? errno : 0;
 	git_buf *buf = &GIT_GLOBAL->error_buf;
 
 	git_buf_clear(buf);
-	if (string) {
-		va_start(arglist, string);
-		git_buf_vprintf(buf, string, arglist);
-		va_end(arglist);
-
+	if (fmt) {
+		git_buf_vprintf(buf, fmt, ap);
 		if (error_class == GIT_ERROR_OS)
 			git_buf_PUTS(buf, ": ");
 	}

src/errors.h

@@ -14,8 +14,8 @@
 /*
  * Set the error message for this thread, formatting as needed.
  */
-
-void git_error_set(int error_class, const char *string, ...) GIT_FORMAT_PRINTF(2, 3);
+void git_error_set(int error_class, const char *fmt, ...) GIT_FORMAT_PRINTF(2, 3);
+void git_error_vset(int error_class, const char *fmt, va_list ap);
 
 /**
  * Set the error message for a regex failure, using the internal regex