Merge branch 'ps/config-env-option-with-separate-value'

"git --config-env var=val cmd" weren't accepted (only
--config-env=var=val was).

* ps/config-env-option-with-separate-value:
  git: support separate arg for `--config-env`'s value
  git.txt: fix synopsis of `--config-env` missing the equals sign

diff --cc Documentation/git.txt
Simple merge

diff --cc git.c
Simple merge

diff --cc t/t1300-config.sh
Simple merge