Emit a better error message when a patch is truncated

src/patch/parse.ts

@@ -16,6 +16,15 @@ import type { StructuredPatch } from '../types.js';
 export function parsePatch(uniDiff: string): StructuredPatch[] {
   const diffstr = uniDiff.split(/\n/),
         list: Partial<StructuredPatch>[] = [];
+
+  // If the final line of the patch is missing the trailing newline character
+  // that should be present, we forgive that, but if the final character of
+  // the patch is a newline as expected, we don't consider there to be a
+  // further blank line afterwards. We remove that blank line here:
+  if (diffstr[diffstr.length - 1] == '') {
+    diffstr.pop();
+  }
+
   let i = 0;
 
   // These helper functions identify line types that can appear between files
@@ -494,10 +503,14 @@ export function parsePatch(uniDiff: string): StructuredPatch[] {
 
     // Perform sanity checking
     if (addCount !== hunk.newLines) {
-      throw new Error('Added line count did not match for hunk at line ' + (chunkHeaderIndex + 1));
+      throw new Error(
+        `New line count did not match for hunk at line ${chunkHeaderIndex + 1}; expected ${hunk.newLines} but got ${addCount}`
+      );
     }
     if (removeCount !== hunk.oldLines) {
-      throw new Error('Removed line count did not match for hunk at line ' + (chunkHeaderIndex + 1));
+      throw new Error(
+        `Old line count did not match for hunk at line ${chunkHeaderIndex + 1}; expected ${hunk.oldLines} but got ${removeCount}`
+      );
     }
 
     // Check for extra hunk-body-like lines after the declared line counts

test/patch/parse.js

@@ -666,10 +666,10 @@ more garbage
 
       expect(function() {
         parsePatch('@@ -1 +1,4 @@');
-      }).to['throw']('Added line count did not match for hunk at line 1');
+      }).to['throw']('New line count did not match for hunk at line 1; expected 4 but got 0');
       expect(function() {
         parsePatch('@@ -1,4 +1 @@');
-      }).to['throw']('Removed line count did not match for hunk at line 1');
+      }).to['throw']('Old line count did not match for hunk at line 1; expected 4 but got 0');
     });
 
 
@@ -939,6 +939,22 @@ line3
       expect(() => {parsePatch(patchStr);}).to.throw('Hunk at line 5 contained invalid line line3');
     });
 
+    it('should emit a useful error message if the final hunk of the patch file ends prematurely', () => {
+      const patchStr = `Index: test
+===================================================================
+--- from\theader1
++++ to\theader2
+@@ -1,4 +1,5 @@
+ line2
+ line3
++line4
+ line5`;
+      // eslint-disable-next-line dot-notation
+      expect(() => { parsePatch(patchStr); }).to.throw(
+        'New line count did not match for hunk at line 5; expected 5 but got 4'
+      );
+    });
+
     it('should parse a single-file `diff --git` patch', function() {
       expect(parsePatch(
 `diff --git a/file.txt b/file.txt