Fix diff skipping lines (#13157)

* Fix diff skipping lines

Backport #13154

ParsePatch previously just skipped all lines that start with "+++ " or "--- "
and makes no attempt to see these lines in context.

This PR rewrites ParsePatch to pay attention to context and position
within a patch, ensuring that --- and +++ are only skipped if
appropriate.

This PR also fixes several issues with incomplete files.

Fix https://codeberg.org/Codeberg/Community/issues/308
Fix #13153

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Add testcase

Signed-off-by: Andrew Thornton <art27@cantab.net>

* fix comment

* simplify error handling

Signed-off-by: Andrew Thornton <art27@cantab.net>

* never return io.EOF

Signed-off-by: Andrew Thornton <art27@cantab.net>

services/gitdiff/gitdiff_test.go

@@ -148,6 +148,27 @@ rename to a b/a a/file b/b file
 			oldFilename: "a b/file b/a a/file",
 			filename:    "a b/a a/file b/b file",
 		},
+		{
+			name: "minuses-and-pluses",
+			gitdiff: `diff --git a/minuses-and-pluses b/minuses-and-pluses
+index 6961180..9ba1a00 100644
+--- a/minuses-and-pluses
++++ b/minuses-and-pluses
+@@ -1,4 +1,4 @@
+--- 1st line
+-++ 2nd line
+--- 3rd line
+-++ 4th line
++++ 1st line
++-- 2nd line
++++ 3rd line
++-- 4th line
+`,
+			oldFilename: "minuses-and-pluses",
+			filename:    "minuses-and-pluses",
+			addition:    4,
+			deletion:    4,
+		},
 	}
 
 	for _, testcase := range tests {