fix: preserve symlink paths when changing directory

Problem: File paths change from symlink to target path after :cd command
when editing files through symbolic links

Solution: Add '~' flag to 'cpoptions' to control symlink resolution.
When included (default), symlinks are resolved maintaining backward
compatibility. When excluded, symlinks are preserved providing the
improved behavior.

runtime/doc/options.txt

@@ -2390,7 +2390,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 
 						*'cpoptions'* *'cpo'* *cpo*
 'cpoptions' 'cpo'	string	(Vim default: "aABceFsz",
-				 Vi default:  all flags, except "#{|&/\."
+				 Vi default:  all flags, except "#{|&/\.~"
 				 |$VIM_POSIX|:  all flags)
 			global
 	A sequence of single character flags.  When a character is present
@@ -2680,6 +2680,13 @@ A jump table for the options with a short description can be found at |Q_op|.
 			character, the cursor won't move. When not included,
 			the cursor would skip over it and jump to the
 			following occurrence.
+								*cpo-~*
+		~	When included, don't resolve symbolic links when
+			changing directory with |:cd|, |:lcd|, or |:tcd|.
+			This preserves the symbolic link path in buffer names
+			and when displaying the current directory.  When
+			excluded (default), symbolic links are resolved to
+			their target paths.
 
 	POSIX flags.  These are not included in the Vi default value, except
 	when $VIM_POSIX was set on startup. |posix|

src/ex_docmd.c

@@ -8029,7 +8029,7 @@ post_chdir(cdscope_T scope)
     }
 
     last_chdir_reason = NULL;
-    shorten_fnames(TRUE);
+    shorten_fnames(vim_strchr(p_cpo, CPO_NOSYMLINKS) == NULL);
 }
 
 /*

src/option.h

@@ -231,10 +231,11 @@ typedef enum {
 #define CPO_CHDIR	'.'	// don't chdir if buffer is modified
 #define CPO_SCOLON	';'	// using "," and ";" will skip over char if
 				// cursor would not move
+#define CPO_NOSYMLINKS	'~'	// don't resolve symlinks when changing directory
 // default values for Vim, Vi and POSIX
 #define CPO_VIM		"aABceFsz"
 #define CPO_VI		"aAbBcCdDeEfFgHiIjJkKlLmMnoOpPqrRsStuvwWxXyZz$!%*-+<>;"
-#define CPO_ALL		"aAbBcCdDeEfFgHiIjJkKlLmMnoOpPqrRsStuvwWxXyZz$!%*-+<>#{|&/\\.;"
+#define CPO_ALL		"aAbBcCdDeEfFgHiIjJkKlLmMnoOpPqrRsStuvwWxXyZz$!%*-+<>#{|&/\\.;~"
 
 // characters for p_ww option:
 #define WW_ALL		"bshl<>[]~"

src/testdir/test_cd.vim

@@ -252,4 +252,67 @@ func Test_getcwd_actual_dir()
   call chdir(startdir)
 endfunc
 
+func Test_cd_preserve_symlinks()
+  " Test new behavior: preserve symlinks when cpo-=~
+  set cpoptions+=~
+
+  let savedir = getcwd()
+  call mkdir('Xsource', 'R')
+  call writefile(['abc'], 'Xsource/foo.txt', 'D')
+
+  if has("win32")
+    silent !mklink /D Xdest Xsource
+  else
+    silent !ln -s Xsource Xdest
+  endif
+  if v:shell_error
+    call delete('Xsource', 'rf')
+    throw 'Skipped: cannot create symlinks'
+  endif
+
+  edit Xdest/foo.txt
+  let path_before = expand('%')
+  call assert_match('Xdest[/\\]foo\.txt$', path_before)
+
+  cd .
+  let path_after = expand('%')
+  call assert_equal(path_before, path_after)
+  call assert_match('Xdest[/\\]foo\.txt$', path_after)
+
+  bwipe!
+  set cpoptions&
+  call delete('Xdest', 'rf')
+  call delete('Xsource', 'rf')
+  call chdir(savedir)
+endfunc
+
+func Test_cd_symlinks()
+  CheckNotMSWindows
+
+  let savedir = getcwd()
+  call mkdir('Xsource', 'R')
+  call writefile(['abc'], 'Xsource/foo.txt', 'D')
+
+  silent !ln -s Xsource Xdest
+  if v:shell_error
+    call delete('Xsource', 'rf')
+    throw 'Skipped: cannot create symlinks'
+  endif
+
+  edit Xdest/foo.txt
+  let path_before = expand('%')
+  call assert_match('Xdest[/\\]foo\.txt$', path_before)
+
+  cd .
+  let path_after = expand('%')
+  call assert_match('Xsource[/\\]foo\.txt$', path_after)
+  call assert_notequal(path_before, path_after)
+
+  bwipe!
+  set cpoptions&
+  call delete('Xdest', 'rf')
+  call delete('Xsource', 'rf')
+  call chdir(savedir)
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab

src/testdir/test_options.vim

@@ -2273,7 +2273,7 @@ func Test_VIM_POSIX()
     qall
   [CODE]
   if RunVim([], after, '')
-    call assert_equal(['aAbBcCdDeEfFgHiIjJkKlLmMnoOpPqrRsStuvwWxXyZz$!%*-+<>#{|&/\.;',
+    call assert_equal(['aAbBcCdDeEfFgHiIjJkKlLmMnoOpPqrRsStuvwWxXyZz$!%*-+<>#{|&/\.;~',
           \            'AS'], readfile('X_VIM_POSIX'))
   endif
 
@@ -2528,7 +2528,7 @@ func Test_string_option_revert_on_failure()
         \ ['completeopt', 'popup', 'a123'],
         \ ['completepopup', 'width:20', 'border'],
         \ ['concealcursor', 'v', 'xyz'],
-        \ ['cpoptions', 'HJ', '~'],
+        \ ['cpoptions', 'HJ', 'Q'],
         \ ['cryptmethod', 'zip', 'a123'],
         \ ['cursorlineopt', 'screenline', 'a123'],
         \ ['debug', 'throw', 'a123'],