We can use latexdiff to compare two LaTeX files. This Perl script has an issue with bibliography files, so place this wrapper before TeX Live's latexdiff:

#!/bin/sh
$HOME/opt/texlive/current/bin/x86_64-linux/latexdiff "$@" |
        sed -E 's/\\DIFadd\{(natexlab)\}/\1/g'

To compare two LaTeX files:

latexdiff file1.tex file2.tex > diff.tex

To compare two main files that input other files:

latexdiff --flatten main1.tex main2.tex > diff.tex

To compare a git revision and main.tex:

latexdiff-vc --git --flatten -r REVISION main.tex
# main-diffREVISION.tex is generated

To compare two git revisions of main.tex:

latexdiff-vc --git --flatten -r REVISION1 -r REVISION2 main.tex
# main-diffREVISION1-REVISION2.tex is generated