Introduce new function real_path_if_valid()

The function is like real_path(), except that it returns NULL on error
instead of dying.
Signed-off-by: NMichael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: NJeff King <peff@peff.net>

abspath.c

@@ -153,6 +153,11 @@ const char *real_path(const char *path)
 	return real_path_internal(path, 1);
 }
 
+const char *real_path_if_valid(const char *path)
+{
+	return real_path_internal(path, 0);
+}
+
 static const char *get_pwd_cwd(void)
 {
 	static char cwd[PATH_MAX + 1];

cache.h

@@ -714,6 +714,7 @@ static inline int is_absolute_path(const char *path)
 }
 int is_directory(const char *);
 const char *real_path(const char *path);
+const char *real_path_if_valid(const char *path);
 const char *absolute_path(const char *path);
 const char *relative_path(const char *abs, const char *base);
 int normalize_path_copy(char *dst, const char *src);