load_large_diff generates incorrect patch if old file does not end in a newline

[MarkRx]

If part of a diff doesn't end in a newline (such as a file not ending in a newline) then the generated patch in load_large_diff might mash two lines together into one line.

Example:

patch = load_large_diff("test.py",
"""\
old content 1
some new content
another line
""",
"""
old content 1
old content 2""")

print(patch)

output (observe the line -old content 2+some new content):

--- 
+++ 
@@ -1,3 +1,3 @@
-
 old content 1
-old content 2+some new content
+another line

[mrT23]

see #1439