\documentclass[varwidth]{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows}
\usepackage[active,pdftex,tightpage]{preview}
\PreviewEnvironment[{[]}]{tikzpicture}

\begin{document}

\tikzset{%
    git_ref/.style = {rectangle, draw=black, align=center, minimum height = 0.75cm},
    git_commit/.style = {circle, draw=black, align=center, minimum size = 1.5cm},
    git_commit_message/.style = {},
    git_arrow/.style = {->,thick},
    git_ref_arrow/.style = {->,thick,dotted}
}
\input{tikz-history}

\end{document}

\documentclass[varwidth]{standalone}
\usepackage{tikz}
\usetikzlibrary{shapes,arrows}
\usepackage[active,pdftex,tightpage]{preview}
\PreviewEnvironment[{[]}]{tikzpicture}

\begin{document}

\tikzset{%
    git_ref/.style = {rectangle, draw=black, align=center},
    git_commit/.style = {circle, draw=black, align=center},
    git_commit_message/.style = {},
    git_arrow/.style = {->,thick},
    git_ref_arrow/.style = {->,thick,dotted}
}
\input{tikz-history}

\end{document}

Allow style to resize around labels
Allow style to resize around labels
