git-gui: Internally allow fetch without storing for future pull support

git-gui: Internally allow fetch without storing for future pull support

This is actually just an underlying code improvement that has no user
visible component yet.  UI improvements to actually fetch and merge via
an arbitrary remote with no tracking branches must still follow to make
this change useful for the end-user.

Our tracking branch specifications are a Tcl list of three components:

  - local tracking branch name
  - remote name/url
  - remote branch name/tag name

This change just makes the first element optional.  If it is an empty
string we will run the fetch, but have the value be saved only into the
special .git/FETCH_HEAD, where we can pick it up and use it for this one
time operation.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>