feat(vcs): Support several git repositories in one project (multi-root)

Only the repository containing the project root was known, so nested clones and
submodules (or a folder of side-by-side service repositories) showed no changes and
could not be committed or pushed. The project now has IDEA's VCS roots: the enclosing
repository plus those found below the project (scanned in the background, rescanned when
a .git appears or goes). Status covers all of them; path-based commands go to the file's
repository and repository-scoped ones take the selected repository.

The Commit view groups changes by repository and commits each one with included changes;
the branch widget names the selected repository and switches between them; Update Project
and Fetch run in every repository; Push lists every repository with outgoing commits; a
repository that stops in a merge or rebase is selected so Continue / Abort act on it.

Closes #12

src/features/vcs/BranchWidget.module.css

@@ -18,6 +18,48 @@
   overflow: hidden;
   text-overflow: ellipsis;
 }
+.repo {
+  flex: none;
+  max-width: 120px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  color: var(--fg-muted);
+}
+.repo::after {
+  content: " /";
+  color: var(--fg-subtle);
+}
+.repos {
+  flex: none;
+  display: flex;
+  flex-wrap: wrap;
+  gap: 4px;
+  padding: 6px 8px;
+  border-bottom: 1px solid var(--border-popup);
+}
+.repoButton {
+  max-width: 160px;
+  height: 22px;
+  padding: 0 8px;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  border: none;
+  border-radius: var(--radius-small);
+  background: transparent;
+  color: var(--fg-secondary);
+  font: inherit;
+  font-size: var(--fs-small);
+  cursor: pointer;
+}
+.repoButton:hover {
+  background: var(--bg-hover);
+}
+.repoSelected,
+.repoSelected:hover {
+  background: var(--bg-selected);
+  color: var(--fg);
+}
 .operation {
   color: var(--warning);
   font-size: var(--fs-tiny);