git-gui: Adapt discovery of oguilib to execdir 'libexec/git-core'

The new execdir is two levels below the root directory, while
the old execdir 'bin' was only one level below.  This commit
adapts the discovery of oguilib that uses relative paths
accordingly. We determine whether we have the extra level in the same
way in which the Makefile defines sharedir, i.e. whether the last
directory part is 'git-core'.

Inspired-by: Steffen Prohaska <prohaska@zib.de>
Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

git-gui.sh

@@ -52,7 +52,11 @@ catch {rename send {}} ; # What an evil concept...
 set oguilib {@@GITGUI_LIBDIR@@}
 set oguirel {@@GITGUI_RELATIVE@@}
 if {$oguirel eq {1}} {
-	set oguilib [file dirname [file dirname [file normalize $argv0]]]
+	set oguilib [file dirname [file normalize $argv0]]
+	if {[file tail $oguilib] eq {git-core}} {
+		set oguilib [file dirname $oguilib]
+	}
+	set oguilib [file dirname $oguilib]
 	set oguilib [file join $oguilib share git-gui lib]
 	set oguimsg [file join $oguilib msgs]
 } elseif {[string match @@* $oguirel]} {