git-gui: Paper bag fix missing translated strings

git-gui: Paper bag fix missing translated strings

The Tcl expression "[append [mc Foo] Bar]" does not return the string
"FooBar" after translation; instead it is setting the variable Foo to
the value Bar, or if Foo is already defined it is appending Bar onto
the end of it.  This is *not* what we wanted to have happen here.

Tcl's join function is actually the correct function but its default
joinStr argument is a single space.  Unfortunately all of our call
sites do not want an extra space added to their string.  So we need
a small wrapper function to make the call to join with an empty
join string.  In C this is (roughly) the job of the strcat function.
Since strcat is not yet used at the global level it is a reasonable
name to use here.

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