Add per-file line counts to the source control sidebar (#2865)

* Add per-file line counts to the source control sidebar

Show +N/-N (green/red) next to each file in the Changes, Untracked, and
Committed-on-branch sections so the magnitude of a change is visible at a
glance. Counts are computed per staging area via `git diff --numstat`;
untracked/new files count their full contents as additions and binary
files show no count.

Consolidate numstat parsing and binary-buffer detection into shared
modules reused by the local status path, the SSH/relay path, and the
existing branch-compare code. Include added/removed in the status-entry
equality check so the sidebar doesn't re-render on unchanged polls.

* fix: harden source control line counts

---------

Co-authored-by: Neil <4138956+nwparker@users.noreply.github.com>

src/shared/binary-buffer.ts

@@ -0,0 +1,12 @@
+// A NUL byte in the first chunk is git's own heuristic for "this is binary".
+const BINARY_SNIFF_BYTES = 8192
+
+export function isBinaryBuffer(buffer: Buffer): boolean {
+  const len = Math.min(buffer.length, BINARY_SNIFF_BYTES)
+  for (let i = 0; i < len; i += 1) {
+    if (buffer[i] === 0) {
+      return true
+    }
+  }
+  return false
+}