"git_path()" returns a static pathname pointer into the git directory
using a printf-like format specifier.
"head_ref()" works like "for_each_ref()", except for just the HEAD.
It returns the result SHA1 on stdout, so you can do
remote=$(git-fetch-pack host:dir branchname)
and it will unpack the objects and "remote" will be the SHA1 name of the
branch on the other side. You can then save that off, or merge it, or
whatever.
It's meant to be used by "git fetch" for the local and ssh case.
It doesn't actually do the fetching now, but it does discover the common
commit point.
