init: change --manifest-depth default to 1

Most users do not care about the manifest history in .repo/manifests/.
Let's change the default to 1 so things work smoothly for most people
most of the time.  For the rare folks who want the full history, they
can add --manifest-depth=0 to their `repo init`.

This has no effect on existing checkouts.

Spot checking Android & CrOS manifests shows significant speedups.
Full history can take O(10's seconds) to O(minutes) while depth of 1
takes constant time of O(~5 seconds).

Bug: 468033850
Change-Id: I4b8ed62a8a636babcc5226552badb69600d0c353
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/535481
Tested-by: Mike Frysinger <vapier@google.com>
Commit-Queue: Mike Frysinger <vapier@google.com>
Reviewed-by: Gavin Mak <gavinmak@google.com>

man/repo-init.1

@@ -1,5 +1,5 @@
 .\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
-.TH REPO "1" "September 2024" "repo init" "Repo Manual"
+.TH REPO "1" "December 2025" "repo init" "Repo Manual"
 .SH NAME
 repo \- repo init - manual page for repo init
 .SH SYNOPSIS
@@ -53,7 +53,7 @@ create a git checkout of the manifest repo
 .TP
 \fB\-\-manifest\-depth\fR=\fI\,DEPTH\/\fR
 create a shallow clone of the manifest repo with given
-depth (0 for full clone); see git clone (default: 0)
+depth (0 for full clone); see git clone (default: 1)
 .SS Manifest (only) checkout options:
 .TP
 \fB\-c\fR, \fB\-\-current\-branch\fR