git_fopen: fix a sparse 'not declared' warning

If git is built with the FREAD_READS_DIRECTORIES build variable set, this
would cause sparse to issue a 'not declared, should it be static?' warning
on Linux. This is a result of the method employed by 'compat/fopen.c' to
suppress the (possible) redefinition of the (system) fopen macro, which
also removes the extern declaration of the git_fopen function.

In order to suppress the warning, introduce a new macro to suppress the
definition (or possibly the re-definition) of the fopen symbol as a macro
override. This new macro (SUPPRESS_FOPEN_REDEFINITION) is only defined in
'compat/fopen.c', just prior to the inclusion of the 'git-compat-util.h'
header file.
Signed-off-by: NRamsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

compat/fopen.c

 /*
  *  The order of the following two lines is important.
  *
- *  FREAD_READS_DIRECTORIES is undefined before including git-compat-util.h
+ *  SUPPRESS_FOPEN_REDEFINITION is defined before including git-compat-util.h
  *  to avoid the redefinition of fopen within git-compat-util.h. This is
  *  necessary since fopen is a macro on some platforms which may be set
  *  based on compiler options. For example, on AIX fopen is set to fopen64
  *  when _LARGE_FILES is defined. The previous technique of merely undefining
  *  fopen after including git-compat-util.h is inadequate in this case.
  */
-#undef FREAD_READS_DIRECTORIES
+#define SUPPRESS_FOPEN_REDEFINITION
 #include "../git-compat-util.h"
 
 FILE *git_fopen(const char *path, const char *mode)

git-compat-util.h

@@ -689,10 +689,12 @@ char *gitstrdup(const char *s);
 #endif
 
 #ifdef FREAD_READS_DIRECTORIES
-#ifdef fopen
-#undef fopen
-#endif
-#define fopen(a,b) git_fopen(a,b)
+# if !defined(SUPPRESS_FOPEN_REDEFINITION)
+#  ifdef fopen
+#   undef fopen
+#  endif
+#  define fopen(a,b) git_fopen(a,b)
+# endif
 extern FILE *git_fopen(const char*, const char*);
 #endif