The version of git that comes with RHEL6 is very old. I'll outline steps for compiling the latest git version on RHEL6. Working from /usr/local/src.

Following instructions for Git Pro book Getting Started Installing Git.

Prerequisites

yum install gcc curl-devel expat-devel gettext-devel openssl-devel zlib-devel perl-ExtUtils-MakeMaker

Optional man page prereqs.

yum install asciidoc xmlto

If using Debian derivative...

apt-get install libcurl4-gnutls-dev libexpat1-dev gettext libz-dev libssl-dev asciidoc xmlto

Checkout and compile

cd /usr/local/src
git clone https://github.com/git/git.git
cd git
#old method
#git ls-remote origin | grep tags
#git checkout v1.8.4
#checkout latest stable version
git checkout $(git ls-remote --tags origin | grep -oE 'v[0-9]+(.[0-9]+){2}$' | tail -n1)
export DEFAULT_HELP_FORMAT="man"
autoconf
./configure
make && make install

Optionally install man pages (recommended).

make man && make install-man

Edit /etc/profile

export PATH="/usr/local/bin:${PATH}"

Upgrading git

cd /usr/local/src/git/
git reset --hard
git clean -xfd
git ls-remote | grep tags
git fetch
#checkout latest stable version
git checkout $(git ls-remote origin | grep 'refs/tags/v[0-9].[0-9].[0-9]$' | tail -n1 | cut -d'/' -f3)
autoconf && ./configure && make && make install && make man && make install-man

Oneliner to install the latest stable git version from git source. Assumes building and installing man pages as well.

cd /usr/local/src/git/ && git reset --hard && git clean -xfd && git fetch && git checkout $(git ls-remote --tags origin | grep -oE 'v[0-9]+(.[0-9]+){2}$' | tail -n1) && autoconf && ./configure && make && make install && make man && make install-man