Use nonrelative paths instead of absolute paths for cloned repositories

Particularly for the "alternates" file, if one will be created, we
want a path that doesn't depend on the current directory, but we want
to retain any symlinks in the path as given and any in the user's view
of the current directory when the path was given.

Signed-off-by: Daniel Barkalow <barkalow@iabervon.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -524,6 +524,7 @@ static inline int is_absolute_path(const char *path)
 	return path[0] == '/';
 }
 const char *make_absolute_path(const char *path);
+const char *make_nonrelative_path(const char *path);
 
 /* Read and unpack a sha1 file into memory, write memory to a sha1 file */
 extern int sha1_object_info(const unsigned char *, unsigned long *);