Add --fetch-branches and --fetch-tags options

[abitmore]

And fix --skip-fetch .

Follow-up of #238.

Features:

• able to fetch a commit that is not in a branch or in the history of a tag (main feature of Specifically fetch the requested commit #238)
  • by default do not fetch additional data (feature of Specifically fetch the requested commit #238)
  • able to fetch additional data (tags or branches) for later use if related option is specified (for Checking out source as tobuild is not ideal #243)
• able to fetch a tag only (addresses Specifically fetch the requested commit #238 (comment))
• git describe --tags works when --fetch-tags is specified (fixes Checking out source as tobuild is not ideal #243)
• always checkout the desired commit/tag/branch even if --skip-fetch is specified (fixes a bug)

@devrandom please review. Thanks.

[devrandom]

Sorry for the delay, missed the notification. Will review this week.

[softminus]

Hey, would you be up to reviewing this PR? The current way gitian fetches git repos causes an issue (described here) with the git describe output on more recent versions of git. git on debian buster doesn't run into this issue, but the git version on debian bullseye does, as shown below.

sasha_work@sasha-vm:~/tmp/zcash-gitian-fetch$ git init
sasha_work@sasha-vm:~/tmp/zcash-gitian-fetch$ git fetch -f --update-head-ok https://github.com/zcash/zcash v4.6.0-2:refs/tags/tobuild 
sasha_work@sasha-vm:~/tmp/zcash-gitian-fetch$ git checkout -q tobuild
sasha_work@sasha-vm:~/tmp/zcash-gitian-fetch$ git submodule update --init --recursive --force
sasha_work@sasha-vm:~/tmp/zcash-gitian-fetch$ git --version; git describe
git version 2.30.2
warning: tag 'tobuild' is externally known as 'v4.6.0-2'
v4.6.0-2-0-gac0c24cfa
sasha_work@sasha-vm:~/tmp/zcash-gitian-fetch$ ~/bin/tmp/usr/bin/git --version; ~/bin/tmp/usr/bin/git describe
git version 2.20.1
warning: tag 'v4.6.0-2' is really 'tobuild' here
v4.6.0-2
sasha_work@sasha-vm:~/tmp/zcash-gitian-fetch$ 

This causes commit hashes to end up in the zcash version string when they shouldn't be going there.

This PR's fetch commands (when run with --fetch-tags) don't run into that issue:

sasha_work@sasha-vm:~/tmp/proposed-fetch-method$ git init
Initialized empty Git repository in /home/sasha_work/tmp/proposed-fetch-method/.git/
sasha_work@sasha-vm:~/tmp/proposed-fetch-method$ git fetch -f --update-head-ok https://github.com/zcash/zcash +refs/tags/*:refs/tags/* 2>/dev/null 
sasha_work@sasha-vm:~/tmp/proposed-fetch-method$ git fetch -f --update-head-ok https://github.com/zcash/zcash v4.6.0-2
From https://github.com/zcash/zcash
 * tag                   v4.6.0-2   -> FETCH_HEAD
sasha_work@sasha-vm:~/tmp/proposed-fetch-method$ git checkout -q FETCH_HEAD
sasha_work@sasha-vm:~/tmp/proposed-fetch-method$ git submodule update --init --recursive --force
sasha_work@sasha-vm:~/tmp/proposed-fetch-method$ ~/bin/tmp/usr/bin/git --version; ~/bin/tmp/usr/bin/git describe
git version 2.20.1
v4.6.0-2
sasha_work@sasha-vm:~/tmp/proposed-fetch-method$ git --version ; git describe
git version 2.30.2
v4.6.0-2
sasha_work@sasha-vm:~/tmp/proposed-fetch-method$ 

[devrandom]

Thank you for the thorough test @abitmore . I think one of the issues with the existing code is that tobuild is created as a tag rather than a branch.

Your solution seems fine.

[devrandom]

just noting that the --update-head-ok here is what makes FETCH_HEAD unconditionally update here. without it, FETCH_HEAD will usually be blank.