diffcol.sh: Use /bin/bash explicitly

diffcol.sh: Use /bin/bash explicitly

Recent Ubuntu uses dash (Debian Almquist Shell; a lightweight POSIX-
compliant shell derived from ash) as /bin/sh by default.  In this case
pager=diffcol.sh doesn't work like this:

> skuribay@ubuntu:~/kernel/linux.git$ stg show
> /home/skuribay/share/stgit/contrib/diffcol.sh: 22: Bad substitution

this is caused by non-bash-compliant /bin/sh.

diffcol.sh is based on Quilt, and Quilt is dependent on bash; Actually
all Quilt scripts use /bin/bash without exceptions.  Then it's good for
diffcol.sh to use /bin/bash.

Signed-off-by: Shinya Kuribayashi <skuribay@ruby.dti.ne.jp>
Signed-off-by: Karl Hasselström <kha@treskal.com>