gc: Introduce new command to remove old projects

When projects are removed from manifest, they are only removed from
worktree and not from .repo/projects and .repo/project-objects. Keeping
data under .repo can be desired if user expects deleted projects to be
restored (e.g. checking out a release branch).

Android has ongoing effort to remove many stale projects and this change
allows users to easily free-up their disk space.

Bug: b/344018971
Bug: 40013312
Change-Id: Id23c7524a88082ee6db908f9fd69dcd5d0c4f681
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/445921
Reviewed-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@chromium.org>
Reviewed-by: Gavin Mak <gavinmak@google.com>
Tested-by: Josip Sokcevic <sokcevic@chromium.org>

man/repo-gc.1

@@ -0,0 +1,43 @@
+.\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
+.TH REPO "1" "December 2024" "repo gc" "Repo Manual"
+.SH NAME
+repo \- repo gc - manual page for repo gc
+.SH SYNOPSIS
+.B repo
+\fI\,gc\/\fR
+.SH DESCRIPTION
+Summary
+.PP
+Cleaning up internal repo state.
+.SH OPTIONS
+.TP
+\fB\-h\fR, \fB\-\-help\fR
+show this help message and exit
+.TP
+\fB\-n\fR, \fB\-\-dry\-run\fR
+do everything except actually delete
+.TP
+\fB\-y\fR, \fB\-\-yes\fR
+answer yes to all safe prompts
+.SS Logging options:
+.TP
+\fB\-v\fR, \fB\-\-verbose\fR
+show all output
+.TP
+\fB\-q\fR, \fB\-\-quiet\fR
+only show errors
+.SS Multi\-manifest options:
+.TP
+\fB\-\-outer\-manifest\fR
+operate starting at the outermost manifest
+.TP
+\fB\-\-no\-outer\-manifest\fR
+do not operate on outer manifests
+.TP
+\fB\-\-this\-manifest\-only\fR
+only operate on this (sub)manifest
+.TP
+\fB\-\-no\-this\-manifest\-only\fR, \fB\-\-all\-manifests\fR
+operate on this manifest and its submanifests
+.PP
+Run `repo help gc` to view the detailed manual.