Add is_absolute_path() and make_absolute_path()

This patch adds convenience functions to work with absolute paths.
The function is_absolute_path() should help the efforts to integrate
the MinGW fork.

Note that make_absolute_path() returns a pointer to a static buffer.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

test-absolute-path.c

@@ -0,0 +1,11 @@
+#include "cache.h"
+
+int main(int argc, char **argv)
+{
+	while (argc > 1) {
+		puts(make_absolute_path(argv[1]));
+		argc--;
+		argv++;
+	}
+	return 0;
+}