git-gui: Correct installation of library to be $prefix/share

We always wanted the library for git-gui to install into the
$prefix/share directory, not $prefix/libexec/share.  All of
the files in our library are platform independent and may
be reused across systems, like any other content stored in
the share directory.

Our computation of where our library should install to was broken
when git itself started installing to $prefix/libexec/git-core,
which was one level down from where we expected it to be.

Signed-off-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

Makefile

@@ -34,8 +34,12 @@ ifndef gitexecdir
 endif
 
 ifndef sharedir
+ifeq (git-core,$(notdir $(gitexecdir)))
+	sharedir := $(dir $(patsubst %/,%,$(dir $(gitexecdir))))share
+else
 	sharedir := $(dir $(gitexecdir))share
 endif
+endif
 
 ifndef INSTALL
 	INSTALL = install