brew-formula-version-picker.gist

brew update
brew versions FORMULA
cd `brew --prefix`
git checkout HASH Library/Formula/FORMULA.rb  # use output of "brew versions"
brew install FORMULA
brew switch FORMULA VERSION
git checkout -- Library/Formula/FORMULA.rb    # reset formula

## Example: Switch to git 1.9.0 in particular:
#
# $ brew versions git
## [ignore the warning about "brew-versions" being unsupported; it works well today]
# 1.9.1    git checkout 6801305 Library/Formula/git.rb
# 1.9.0    git checkout e2d162d Library/Formula/git.rb
# 1.8.5.5  git checkout 2fe5763 Library/Formula/git.rb
# 1.8.5.4  git checkout 5c6cb18 Library/Formula/git.rb
## ... etc ...
#
# $ cd `brew --prefix`
#
## copy/paste the 1.9.0 line above like so:
# $ git checkout e2d162d Library/Formula/git.rb
#
# $ brew install git
#
# $ brew switch git 1.9.0
#
# $ git checkout -- Library/Formula/git.rb
#
# $ git --version
# git version 1.9.0
#
## DONE :)

After this step:
$ git checkout e2d162d Library/Formula/git.rb

I had to "brew unlink git" before installing.

I didn't have to:
$ brew switch git 1.9.0
$ git checkout -- Library/Formula/git.rb

Thanks Michael! Yeah, doing what brew says to do along the way is... the right thing to do.
Note if you have multiple versions of a formula installed, "brew link" may not work, hence "brew switch".
(Maybe you hadn't previously used brew to manage your git?) YMMV.
I'm not a homebrew expert, just sharing what worked for me. :)