Add internal common.h file

This one pulls in compiler compatibility macros, some
common header files, and also the public common.h header.

C source files are modified to use the private common.h
in favour of the public one.

Signed-off-by: Andreas Ericsson <ae@op5.se>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

src/common.h

+#ifndef INCLUDE_common_h__
+#define INCLUDE_common_h__
+
+#include "cc-compat.h"
+#include <errno.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+
+#include "git/common.h"
+#endif /* INCLUDE_common_h__ */