fix: remove trailing slashes before making directory

Remove the trailing slashes from 'undofile' and 'backupdir' before
creating directories. They cause problems on Windows which doesn't
recognize these slashes as proper path separators.

src/nvim/fileio.c

@@ -2688,7 +2688,12 @@ buf_write(
         /*
          * Isolate one directory name, using an entry in 'bdir'.
          */
-        (void)copy_option_part(&dirp, IObuff, IOSIZE, ",");
+        size_t dir_len = copy_option_part(&dirp, IObuff, IOSIZE, ",");
+        p = IObuff + dir_len;
+        bool trailing_pathseps = after_pathsep((char *)IObuff, (char *)p) && p[-1] == p[-2];
+        if (trailing_pathseps) {
+          IObuff[dir_len - 2] = NUL;
+        }
         if (*dirp == NUL && !os_isdir(IObuff)) {
           int ret;
           char *failed_dir;
@@ -2698,8 +2703,7 @@ buf_write(
             xfree(failed_dir);
           }
         }
-        p = IObuff + STRLEN(IObuff);
+        if (trailing_pathseps) {
-        if (after_pathsep((char *)IObuff, (char *)p) && p[-1] == p[-2]) {
           // Ends with '//', Use Full path
           if ((p = (char_u *)make_percent_swname((char *)IObuff, (char *)fname))
               != NULL) {
@@ -2849,7 +2853,12 @@ nobackup:
         /*
          * Isolate one directory name and make the backup file name.
          */
-        (void)copy_option_part(&dirp, IObuff, IOSIZE, ",");
+        size_t dir_len = copy_option_part(&dirp, IObuff, IOSIZE, ",");
+        p = IObuff + dir_len;
+        bool trailing_pathseps = after_pathsep((char *)IObuff, (char *)p) && p[-1] == p[-2];
+        if (trailing_pathseps) {
+          IObuff[dir_len - 2] = NUL;
+        }
         if (*dirp == NUL && !os_isdir(IObuff)) {
           int ret;
           char *failed_dir;
@@ -2859,8 +2868,7 @@ nobackup:
             xfree(failed_dir);
           }
         }
-        p = IObuff + STRLEN(IObuff);
+        if (trailing_pathseps) {
-        if (after_pathsep((char *)IObuff, (char *)p) && p[-1] == p[-2]) {
           // path ends with '//', use full path
           if ((p = (char_u *)make_percent_swname((char *)IObuff, (char *)fname))
               != NULL) {

src/nvim/memline.c

@@ -1442,7 +1442,7 @@ recover_names (
  * Append the full path to name with path separators made into percent
  * signs, to dir. An unnamed buffer is handled as "" (<currentdir>/"")
  */
-char *make_percent_swname(const char *dir, char *name)
+char *make_percent_swname(const char *dir, const char *name)
   FUNC_ATTR_NONNULL_ARG(1)
 {
   char *d = NULL;

src/nvim/undo.c

@@ -672,6 +672,7 @@ char *u_get_undo_file_name(const char *const buf_ffname, const bool reading)
 #ifdef HAVE_READLINK
   char fname_buf[MAXPATHL];
 #endif
+  char *p;
 
   if (ffname == NULL) {
     return NULL;
@@ -704,6 +705,13 @@ char *u_get_undo_file_name(const char *const buf_ffname, const bool reading)
       memmove(tail + tail_len + 1, ".un~", sizeof(".un~"));
     } else {
       dir_name[dir_len] = NUL;
+
+      // Remove trailing pathseps from directory name
+      p = &dir_name[dir_len - 1];
+      while (vim_ispathsep(*p)) {
+        *p-- = NUL;
+      }
+
       bool has_directory = os_isdir((char_u *)dir_name);
       if (!has_directory && *dirp == NUL && !reading) {
         // Last directory in the list does not exist, create it.
@@ -720,7 +728,7 @@ char *u_get_undo_file_name(const char *const buf_ffname, const bool reading)
       if (has_directory) {
         if (munged_name == NULL) {
           munged_name = xstrdup(ffname);
-          for (char *p = munged_name; *p != NUL; MB_PTR_ADV(p)) {
+          for (p = munged_name; *p != NUL; MB_PTR_ADV(p)) {
             if (vim_ispathsep(*p)) {
               *p = '%';
             }