strncat() has the same quadratic behavior as strcat() and is
difficult-to-read and bug-prone.  While it hasn't yet been a
problem in git iself, strncat() found it's way into 'master'
of cgit and caused segfaults on my system.
Signed-off-by: NEric Wong <e@80x24.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>