shada: Fix jump/change list merging code

Errors happens under following conditions:

1. Jump/change list is full.
2. New jump/change list item should go between some of the old ones.

src/nvim/shada.c

@@ -1213,10 +1213,10 @@ static inline bool marks_equal(const pos_T a, const pos_T b)
     if (i > 0) { \
       if (jl_len == JUMPLISTSIZE) { \
         free_func(jumps[0]); \
-        if (i == JUMPLISTSIZE) { \
-          i = JUMPLISTSIZE - 1; \
-        } \
+        i--; \
+        if (i > 0) { \
           memmove(&jumps[0], &jumps[1], sizeof(jumps[1]) * (size_t) i); \
+        } \
       } else if (i != jl_len) { \
         memmove(&jumps[i + 1], &jumps[i], \
                 sizeof(jumps[0]) * (size_t) (jl_len - i)); \

test/functional/shada/merging_spec.lua

@@ -990,4 +990,46 @@ describe('ShaDa changes support code', function()
     end
     eq(found, 100)
   end)
+
+  it('merges JUMPLISTSIZE changes when writing, with new items between old',
+  function()
+    nvim_command('edit /a/b/c')
+    nvim_command('keepjumps call setline(1, range(202))')
+    local shada = ''
+    for i = 1,101 do
+      local t = i * 2
+      shada = shada .. (
+          '\011\204%c\019\131\162mX\195\161f\196\006/a/b/c\161l\204%c'
+          ):format(t, t)
+    end
+    wshada(shada)
+    eq(0, exc_exec(sdrcmd()))
+    local shada = ''
+    for i = 1,100 do
+      shada = shada .. ('\011%c\018\131\162mX\195\161f\196\006/a/b/c\161l%c'
+                       ):format(i, i)
+    end
+    local changes = {}
+    for i = 1, 100 do
+      changes[i] = {line=i}
+    end
+    for i = 1, 101 do
+      local t = i * 2
+      changes[(t > #changes + 1) and (#changes + 1) or t] = {line=t}
+    end
+    wshada(shada)
+    eq(0, exc_exec('wshada ' .. shada_fname))
+    local shift = #changes - 100
+    for i = 1,100 do
+      changes[i] = changes[i + shift]
+    end
+    local found = 0
+    for _, v in ipairs(read_shada_file(shada_fname)) do
+      if v.type == 11 and v.value.f == '/a/b/c' then
+        found = found + 1
+        eq(changes[found].line, v.value.l)
+      end
+    end
+    eq(found, 100)
+  end)
 end)