Build: Allow Rebuilding A Particular Commit

[harding]

This PR updates the build script with two minor improvements:

1. When force building, the script no longer exits if you're attempting to the most recently-built commit.
2. The commit ID (commitish) of the most recently built commit is saved for use in other scripts

The intention here is to be able to create a cronjob for PR #976 that will force rebuild the site every night at midnight. Point 1 makes that possible without the hard rebase hack @saivann and I currently use when manually rebuilding and point 2 prevents an automatic force build from allowing someone to bypass the signed commits by pushing a commit to master at exactly the right time.

I also switched the commit.txt command in the Makefile to use the same syntax as the build file, as I thought Saïvann's way was better.

This PR has not been tested, but I will test it when I manually update the file on the build server.

[saivann]

@harding As I understand it, the script would actually checkout to last commit from a repository provided as a command line argument (BUILD_COMMITISH). If so, what repository would you use here? I am not sure that I understand correctly what is in place to prevent unsigned commits to be built if this script is called with -force automatically.

Alternatively, maybe using a "-rebuild" option that does the same as "-auto", but skipping "$LASTLOCALCOMMIT == $LASTREMOTECOMMIT", would add less complexity and risk of messing up with "-force" (at the cost of always needing last commit to be signed).

[harding]

@saivann the command line argument can be a repository or any other commitish, but it's intended to be a particular commit. For example, if I wanted to rebuild the site right now, I would run:

update_site.sh force 09acaafbb0f265934e45f45acf99eb9e2570b787

The cronjob would be:

update_site.sh force $( cat /bitcoin.org/last-successful-build-commitish )

Since that file is only updated after a successful build, that previous build would've had to have either been signed (verified during that build by auto) or manually force built by either you or me.