Add compat/snprintf.c for systems that return bogus

Some systems (namely HPUX and Windows) return -1 when maxsize in snprintf()
and in vsnprintf() is reached. So replace snprintf() and vsnprintf()
functions with our own ones that return correct value upon overflow.

[jc: verified that review comments by J6t have been incorporated, and
 tightened the check to verify the resulting buffer contents, suggested
 by Wayne Davison]

Signed-off-by: Michal Rokos <michal.rokos@nextsoft.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/snprintf.c

@@ -0,0 +1,40 @@
+#include "../git-compat-util.h"
+
+#undef vsnprintf
+int git_vsnprintf(char *str, size_t maxsize, const char *format, va_list ap)
+{
+	char *s;
+	int ret;
+
+	ret = vsnprintf(str, maxsize, format, ap);
+	if (ret != -1)
+		return ret;
+
+	s = NULL;
+	if (maxsize < 128)
+		maxsize = 128;
+
+	while (ret == -1) {
+		maxsize *= 4;
+		str = realloc(s, maxsize);
+		if (! str)
+			break;
+		s = str;
+		ret = vsnprintf(str, maxsize, format, ap);
+	}
+	free(s);
+	return ret;
+}
+
+int git_snprintf(char *str, size_t maxsize, const char *format, ...)
+{
+	va_list ap;
+	int ret;
+
+	va_start(ap, format);
+	ret = git_vsnprintf(str, maxsize, format, ap);
+	va_end(ap);
+
+	return ret;
+}
+