Avoid double-slash in path names that depend on $(sharedir).
Recent git-gui has the ability to determine the location of its library
files relative to the --exec-dir. Its Makefile enables this capability
depending on the install paths that are specified. However, without this
fix there is an extra slash in a path specification, so that the Makefile
does not recognize the equivalence of two paths that it compares.
As a side-effect, this avoids an ugly compiled-in double-slash in
$(template_dir).
Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>