strbuf_readlink(): support link targets that exceed 2*PATH_MAX

The `strbuf_readlink()` function refuses to read link targets that
exceed 2*PATH_MAX (even if a sufficient size was specified by the
caller).

The reason that that limit is 2*PATH_MAX instead of PATH_MAX is that
the symlink targets do not need to be normalized. After running
`ln -s a/../a/../a/../a/../b c`, the target of the symlink `c` will not
be normalized to `b` but instead be much longer. As such, symlink
targets' lengths can far exceed PATH_MAX.

They are frequently much longer than 2*PATH_MAX on Windows, which
actually supports paths up to 32,767 characters, but sets PATH_MAX to
260 for backwards compatibility. For full details, see
https://learn.microsoft.com/en-us/windows/win32/fileio/maximum-file-path-limitation

Let's just hard-code the limit used by `strbuf_readlink()` to 32,767 and
make it independent of the current platform's PATH_MAX.

Based-on-a-patch-by: Karsten Blees <[email protected]>
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

strbuf.c

@@ -566,7 +566,7 @@ ssize_t strbuf_write(struct strbuf *sb, FILE *f)
 	return sb->len ? fwrite(sb->buf, 1, sb->len, f) : 0;
 }
 
-#define STRBUF_MAXLINK (2*PATH_MAX)
+#define STRBUF_MAXLINK (32767)
 
 int strbuf_readlink(struct strbuf *sb, const char *path, size_t hint)
 {