Branch tag gdb_7_3-branch created for insight-specific sources

I've added the gdb_7_4-branch branch tag for the following insight
specific source directories:

    gdb/gdbtk
    gdb/testsuite/gdb.gdbtk
    tcl
    tk
    itcl
    libgui

As usual, I found that I couldn't use the same date as that used by
GDB for creating the branch point.  (I didn't create a branch point,
though I can if desired.)  I ended up using -D 2012-03-28 in order
to end up with a set of sources that would build.

Kevin