git_command: make execution synchronous

Every use of GitCommand in the tree just calls Wait as soon as it's
instantiated.  Move the bulk of the logic into the init path to make
the call synchronous to simplify.  We'll cleanup the users of the
Wait API to follup commits -- having this split makes it easier to
track down regressions.

Change-Id: I1e8c519efa912da723749ff7663558c04c1f491c
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/297244


Reviewed-by: Jonathan Nieder <jrn@google.com>
Tested-by: Mike Frysinger <vapier@google.com>