Add script 'release/update-manpages' to generate manpages

Debian package started to ship manpages for repo since 2.8 [1]
And it's about for one year. So I think it should be upstreamed.

The script depends on help2man, which is available in both debian [2]
and ubuntu [3].

[1] https://tracker.debian.org/news/1150858/accepted-repo-28-1-source-into-unstable
[2] https://tracker.debian.org/pkg/help2man
[3] https://launchpad.net/ubuntu/+source/help2man

Change-Id: Ide2b356d0944ebde34cc96c6d5a782655bd72288
Signed-off-by: Roger Shimizu <rosh@debian.org>
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/309782
Reviewed-by: Mike Frysinger <vapier@google.com>

man/repo-download.1

@@ -0,0 +1,44 @@
+.\" DO NOT MODIFY THIS FILE!  It was generated by help2man.
+.TH REPO "1" "July 2021" "repo download" "Repo Manual"
+.SH NAME
+repo \- repo download - manual page for repo download
+.SH SYNOPSIS
+.B repo
+\fI\,download {\/\fR[\fI\,project\/\fR] \fI\,change\/\fR[\fI\,/patchset\/\fR]\fI\,}\/\fR...
+.SH DESCRIPTION
+Summary
+.PP
+Download and checkout a change
+.SH OPTIONS
+.TP
+\fB\-h\fR, \fB\-\-help\fR
+show this help message and exit
+.TP
+\fB\-b\fR BRANCH, \fB\-\-branch\fR=\fI\,BRANCH\/\fR
+create a new branch first
+.TP
+\fB\-c\fR, \fB\-\-cherry\-pick\fR
+cherry\-pick instead of checkout
+.TP
+\fB\-x\fR, \fB\-\-record\-origin\fR
+pass \fB\-x\fR when cherry\-picking
+.TP
+\fB\-r\fR, \fB\-\-revert\fR
+revert instead of checkout
+.TP
+\fB\-f\fR, \fB\-\-ff\-only\fR
+force fast\-forward merge
+.SS Logging options:
+.TP
+\fB\-v\fR, \fB\-\-verbose\fR
+show all output
+.TP
+\fB\-q\fR, \fB\-\-quiet\fR
+only show errors
+.PP
+Run `repo help download` to view the detailed manual.
+.SH DETAILS
+.PP
+The 'repo download' command downloads a change from the review system and makes
+it available in your project's local working directory. If no project is
+specified try to use current directory as a project.