Cygwin: symlink_native: allow linking to . again

In 827743a (Cygwin: symlink_native: allow linking to `..`,
2025-06-20), I fixed linking to `..` (which had inadvertently
targeted an incorrect location prior to that fix), but inadvertently
broke linking to `.` (which would now try to pass the empty string as
`lpTargetFileName` to `CreateSymbolicLinkW()`, failing with an
`ERROR_INVALID_REPARSE_DATA` which would be surfaced as "Permission
denied").

Let's fix this by special-casing an empty string as path as referring to
the current directory.

Note: It is unclear to me why the `winsymlinks:nativestrict` code path
even tries to simplify the symbolic link's target path (e.g. turn an
absolute path into a relative one). As long as it refers to a regular
Win32 file or directory, I would think that even something like
`././c` should have only the slashes converted, not the path
simplified (i.e. `.\.\c` instead of `c`). But that's a larger
discussion, and I would like to have the bug worked around swiftly.

Fixes: 827743a (Cygwin: symlink_native: allow linking to `..`, 2025-06-20)
Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

winsup/cygwin/path.cc

@@ -1895,7 +1895,10 @@ symlink_native (const char *oldpath, path_conv &win32_newpath)
 	    e_old = wcpcpy (e_old, L"..\\"), num--;
 	  if (num > 0)
 	    e_old = wcpcpy (e_old, L"..");
-	  wcpcpy (e_old, c_old);
+	  if (e_old == final_oldpath->Buffer && c_old[0] == L'\0')
+	    wcpcpy (e_old, L".");
+	  else
+	    wcpcpy (e_old, c_old);
 	}
     }
   /* If the symlink target doesn't exist, don't create native symlink.

winsup/cygwin/release/3.6.5

@@ -29,3 +29,5 @@ Fixes:
 - Fix multi-thread safety of fork()/exec() by adding the same locking as was
   done for spawn.
   Addresses: https://cygwin.com/pipermail/cygwin/2025-September/258801.html
+
+- Fix native symlink to '.' (a regresison in 3.6.4)