brew update fails wtih fatal: could not read Username
Solution 1:
After trying to fix the gitconfig in my local I found out that actually brew was the issue.
For me homebrew-dupes and homebrew-versions caused the problem. But the fix is similar to taps.
Just do
brew tap
which gives you
...
homebrew/dupes
homebrew/versions
...
a simple untap
brew untap homebrew/dupes
brew untap homebrew/versions
after which everything is ok
brew update
Already up-to-date.