Patch to allow vim to use any undo file. Usage: Create a file with many blank lines and use :rundo! path/to/undo/file . Then use u or r to undo/redo

vim8_0_1272_force_rundo.diff

commit 4775e38f78c7990306fd67fa1145433bb4779722
Author: Rob Wu <rob@robwu.nl>
Date:   Thu Nov 16 12:50:10 2017 +0100

    Add :rundo! support
    
    Based on https://groups.google.com/d/msg/vim_use/pOia0eB8ZWo/FmIQ4lUOPDIJ
    
    Written for https://github.com/vim/vim/archive/v8.0.1272.tar.gz

diff --git a/src/ex_cmds.h b/src/ex_cmds.h
index 41c7c80..7b4cff0 100644
--- a/src/ex_cmds.h
+++ b/src/ex_cmds.h
@@ -1208,7 +1208,7 @@ EX(CMD_rubyfile,	"rubyfile",	ex_rubyfile,
 			RANGE|FILE1|NEEDARG|CMDWIN,
 			ADDR_LINES),
 EX(CMD_rundo,		"rundo",	ex_rundo,
-			NEEDARG|FILE1,
+			BANG|NEEDARG|FILE1,
 			ADDR_LINES),
 EX(CMD_rviminfo,	"rviminfo",	ex_viminfo,
 			BANG|FILE1|TRLBAR|CMDWIN,
diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index 547d1ac..b6a2f88 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -9530,7 +9530,7 @@ ex_rundo(exarg_T *eap)
     char_u hash[UNDO_HASH_SIZE];
 
     u_compute_hash(hash);
-    u_read_undo(eap->arg, hash, NULL);
+    u_read_undo(eap->arg, hash, NULL, eap->forceit);
 }
 #endif
 
diff --git a/src/fileio.c b/src/fileio.c
index 34dcdb6..5a40faa 100644
--- a/src/fileio.c
+++ b/src/fileio.c
@@ -2699,7 +2699,7 @@ failed:
 	char_u	hash[UNDO_HASH_SIZE];
 
 	sha256_finish(&sha_ctx, hash);
-	u_read_undo(NULL, hash, fname);
+	u_read_undo(NULL, hash, fname, FALSE);
     }
 #endif
 
diff --git a/src/option.c b/src/option.c
index ceafea3..445ba20 100644
--- a/src/option.c
+++ b/src/option.c
@@ -8134,7 +8134,7 @@ set_bool_option(
 			&& !curbufIsChanged() && curbuf->b_ml.ml_mfp != NULL)
 		{
 		    u_compute_hash(hash);
-		    u_read_undo(NULL, hash, curbuf->b_fname);
+		    u_read_undo(NULL, hash, curbuf->b_fname, FALSE);
 		}
 	    }
 	    curbuf = save_curbuf;
diff --git a/src/proto/undo.pro b/src/proto/undo.pro
index 2b3258d..ba5336b 100644
--- a/src/proto/undo.pro
+++ b/src/proto/undo.pro
@@ -9,7 +9,7 @@ int u_savecommon(linenr_T top, linenr_T bot, linenr_T newbot, int reload);
 void u_compute_hash(char_u *hash);
 char_u *u_get_undo_file_name(char_u *buf_ffname, int reading);
 void u_write_undo(char_u *name, int forceit, buf_T *buf, char_u *hash);
-void u_read_undo(char_u *name, char_u *hash, char_u *orig_name);
+void u_read_undo(char_u *name, char_u *hash, char_u *orig_name, int force);
 void u_undo(int count);
 void u_redo(int count);
 void undo_time(long step, int sec, int file, int absolute);
diff --git a/src/undo.c b/src/undo.c
index 2c5725f..2103bdf 100644
--- a/src/undo.c
+++ b/src/undo.c
@@ -1786,7 +1786,7 @@ theend:
  * "hash[UNDO_HASH_SIZE]" must be the hash value of the buffer text.
  */
     void
-u_read_undo(char_u *name, char_u *hash, char_u *orig_name)
+u_read_undo(char_u *name, char_u *hash, char_u *orig_name, int force)
 {
     char_u	*file_name;
     FILE	*fp;
@@ -1917,8 +1917,9 @@ u_read_undo(char_u *name, char_u *hash, char_u *orig_name)
 	goto error;
     }
     line_count = (linenr_T)undo_read_4c(&bi);
-    if (memcmp(hash, read_hash, UNDO_HASH_SIZE) != 0
+    if ((memcmp(hash, read_hash, UNDO_HASH_SIZE) != 0
 				  || line_count != curbuf->b_ml.ml_line_count)
+	    && !force)
     {
 	if (p_verbose > 0 || name != NULL)
 	{