wrap: Fix diff_files not finding the patch to apply

When `self.wrap.filesdir` is a relative path, which happens when
`meson subprojects update` is run, the path to the patch must be
provided relative to the working directory in which `patch` or `git`
is run.

`self.wrap.filesdir` is absolute when `Resolve()` is invoked by the
Meson interpreter, which is why this wasn't detected by the tests.

mesonbuild/wrap/wrap.py

@@ -647,14 +647,15 @@ def apply_diff_files(self) -> None:
             path = Path(self.wrap.filesdir) / filename
             if not path.exists():
                 raise WrapException(f'Diff file "{path}" does not exist')
+            relpath = os.path.relpath(str(path), self.dirname)
             if PATCH:
-                cmd = [PATCH, '-f', '-p1', '-i', str(path)]
+                cmd = [PATCH, '-f', '-p1', '-i', relpath]
             elif GIT:
                 # If the `patch` command is not available, fall back to `git
                 # apply`. The `--work-tree` is necessary in case we're inside a
                 # Git repository: by default, Git will try to apply the patch to
                 # the repository root.
-                cmd = [GIT, '--work-tree', '.', 'apply', '-p1', str(path)]
+                cmd = [GIT, '--work-tree', '.', 'apply', '-p1', relpath]
             else:
                 raise WrapException('Missing "patch" or "git" commands to apply diff files')