Avoid double-slash in path names that depend on $(sharedir).

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.

A side-effect is that all "standard" builds (which do not set $(sharedir)
explicitly) now exploit above mentioned gut-gui feature.

Another side-effect is that an ugly compiled-in double-slash in
$(template_dir) is avoided.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>