perf(vcs): filter the Git log with git log instead of a background search index (#1537)

* perf(vcs): filter the Git log with git log instead of a background search index

The log built a persistent inverted index of every commit's message, authors and
changed paths in the background, with a progress bar above the commit list, and
walked trees itself to filter by path. Building it took minutes on large
repositories, and filters were slow until it finished.

- Text: plain text uses `git log --fixed-strings --grep` (with
  `--regexp-ignore-case` unless Match Case is on); text of 4+ hex digits also
  matches hash prefixes. Regex filters still match in-process.
- Paths: `git log -- :(literal)<paths>` replaces the in-process path walk and
  its per-commit tree cache. It uses git's history simplification and, when the
  repository has a changed-path commit-graph, its Bloom filters. Glob characters
  in a path are literal.
- Authors match against the commit metadata cache.
- Removed the search tables, `index_log`, the `vcs_log_index` command and the
  indexing progress bar. The commit metadata cache stays.

* docs(contributing): ask for screenshots only when a change alters what users see

Removing a feature together with its UI, copy-only and non-visual changes now
describe what was checked instead of attaching before/after screenshots.

* fix(vcs): match case-insensitive non-ASCII Git log text in process

git folds case only for ASCII under the C locale it runs with, so a
case-insensitive filter like "ärger" missed "Ärger". Such text is now
matched against commit messages directly. Path and text selection also
ignore log.showSignature and log.follow from the user's config.

.github/pull_request_template.md

@@ -9,7 +9,8 @@
 
 <!-- Required. Commands you actually ran and their results; what else this could
      affect and how you checked it still works.
-     UI changes: before/after screenshots. Multi-step interactions: a recording.
+     Visible UI changes: before/after screenshots (not for removed UI or copy-only
+     changes). Multi-step interactions: a recording.
      Backend changes: logs or command output. -->
 
 ## Review focus