Fix the syntax of the if-expression in main
Fix the call for compiling Tup
Implement --compile-only
Do not require --compile when using --compile-only

This corrects a bug by which

    $ ./Gxisdatigu.scm --compile-only

is a no-op.
Update my personal projects when updating Git repos
Fetch from additional remotes when updating personal projects

For some projects I want to fetch from more than the ‘origin’ remote
repository.  This patch allows me to do so.  It first loads SRFI-16
and then rewrites update-git-sub-directories in terms of case-lambda
so that the procedure accepts an optional second argument: the name of
the remote repository to fetch from, defaulting to ‘origin’.  Then the
patch defines a list mapping project directories to the names of
additional remotes I want to fetch.  And finally it adds an additional
loop in main to perform these fetches via update-git-sub-directories.