Refine the build id string

We use git describe to get the SHA1 so that we append -dirty for dirty
worktrees and include the HEAD commit subject and date.

src/Makefile.am

@@ -32,7 +32,7 @@ weston_SOURCES =				\
 	weston-egl-ext.h
 
 git-version.h : .FORCE
-	echo "#define WESTON_SHA1 \"$(shell git --git-dir=$(top_srcdir)/.git show-ref --head --hash=20 HEAD)\"" > $@-new
+	echo "#define BUILD_ID \"$(shell git --work-tree=$(top_srcdir) describe --always --dirty) $(shell git --work-tree=$(top_srcdir) log -1 --format='%s (%ci)')\"" > $@-new
 	cmp -s $@ $@-new || cp $@-new $@
 	rm $@-new
 

src/compositor.c

@@ -3220,7 +3220,7 @@ int main(int argc, char *argv[])
 		   STAMP_SPACE "Bug reports to: %s\n"
 		   STAMP_SPACE "Build: %s\n",
 		   PACKAGE_STRING, PACKAGE_URL, PACKAGE_BUGREPORT,
-		   WESTON_SHA1);
+		   BUILD_ID);
 	log_uname();
 
 	display = wl_display_create();