lockfile: change lock_file::filename into a strbuf

For now, we still make sure to allocate at least PATH_MAX characters
for the strbuf because resolve_symlink() doesn't know how to expand
the space for its return value.  (That will be fixed in a moment.)

Another alternative would be to just use a strbuf as scratch space in
lock_file() but then store a pointer to the naked string in struct
lock_file.  But lock_file objects are often reused.  By reusing the
same strbuf, we can avoid having to reallocate the string most times
when a lock_file object is reused.

Helped-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache.h

@@ -580,7 +580,7 @@ struct lock_file {
 	volatile int fd;
 	volatile pid_t owner;
 	char on_list;
-	char filename[PATH_MAX];
+	struct strbuf filename;
 };
 #define LOCK_DIE_ON_ERROR 1
 #define LOCK_NODEREF 2