tg: tickle gc --auto when appropriate

Any of tg import, tg export or tg update could potentially result
in creation of a fair number of new loose objects as part of their
normal operation.

Run the "git gc --auto" command at the end of these commands to
hint to Git that it's a good time to check and see if any automatic
gc might need to be performed.

Be careful to first clean up any TopGit temporary directory before
running "git gc --auto" to avoid having it hanging around during
any automatic gc operation.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

tg-export.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 # TopGit - A different patch queue manager
 # Copyright (C) 2008 Petr Baudis <pasky@suse.cz>
-# Copyright (C) 2015-2017 Kyle J. McKay <mackyle@gmail.com>
+# Copyright (C) 2015-2018 Kyle J. McKay <mackyle@gmail.com>
 # All rights reserved
 # GPLv2
 
@@ -502,3 +502,7 @@ elif [ "$driver" = "linearize" ]; then
 	fi
 
 fi
+ec=$?
+tmpdir_cleanup || :
+git gc --auto || :
+exit $ec

tg-import.sh

@@ -2,7 +2,7 @@
 # TopGit - A different patch queue manager
 # Copyright (C) 2008 Petr Baudis <pasky@suse.cz>
 # Copyright (C) 2008 Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
-# Copyright (C) 2015,2017 Kyle J. McKay <mackyle@gmail.com>
+# Copyright (C) 2015,2017,2018 Kyle J. McKay <mackyle@gmail.com>
 # All rights reserved.
 # GPLv2
 
@@ -197,3 +197,7 @@ for revpair in $ranges; do
 	}
 	test $? -eq 0
 done
+ec=$?
+tmpdir_cleanup || :
+git gc --auto || :
+exit $ec

tg-update.sh

@@ -1268,3 +1268,7 @@ info "Returning to ${current#refs/heads/}..."
 checkout_symref_full "$current"
 ! [ -f "$git_dir/TGMERGE_MSG" ] || [ -e "$git_dir/MERGE_MSG" ] ||
 	mv -f "$git_dir/TGMERGE_MSG" "$git_dir/MERGE_MSG" || :
+ec=$?
+tmpdir_cleanup || :
+git gc --auto || :
+exit $ec