Normally it's not possible to download just one file from git without downloading the whole repository as suggested in the first answer. It's because Git doesn't store files as you think (as CVS/SVN do), but it generates them based on the entire history of the project.
But there are some workarounds for specific cases. See below:
GitHub
If this file is on github.com, try e.g.:
wget https://raw.githubusercontent.com/user/project/master/README GitWeb
If you're using Git on the Server - GitWeb, then you may try in example (change it into the right path):
wget "http://example.com/gitweb/?p=example;a=blob_plain;f=README.txt;hb=HEAD" GitWeb at drupalcode.org
Example:
wget "http://drupalcode.org/project/ads.git/blob_plain/refs/heads/master:/README.md" googlesource.com
There is an undocumented feature that allows you to download base64-encoded versions of raw files:
curl "https://chromium.googlesource.com/chromium/src/net/+/master/http/transport_security_state_static.json?format=TEXT" | base64 --decode