Add git version requirement in documentation

See https://github.com/ohmyzsh/ohmyzsh/pull/7172#issuecomment-592875226

README.md

@@ -25,7 +25,7 @@ To learn more, visit [ohmyz.sh](https://ohmyz.sh), follow [@ohmyzsh](https://twi
 * A Unix-like operating system: macOS, Linux, BSD. On Windows: WSL is preferred, but cygwin or msys also mostly work.
 * [Zsh](https://www.zsh.org) should be installed (v4.3.9 or more recent). If not pre-installed (run `zsh --version` to confirm), check the following instructions here: [Installing ZSH](https://github.com/ohmyzsh/ohmyzsh/wiki/Installing-ZSH)
 * `curl` or `wget` should be installed
-* `git` should be installed
+* `git` should be installed (recommended v1.7.2 or higher)
 
 ### Basic Installation
 

tools/upgrade.sh

@@ -38,7 +38,7 @@ if [ -n "$remote" ]; then
 fi
 
 printf "${BLUE}%s${NORMAL}\n" "Updating Oh My Zsh"
-if git pull --rebase --autostash --stat origin master
+if git -c rebase.autoStash=true pull --rebase --stat origin master
 then
   printf '%s' "$GREEN"
   printf '%s\n' '         __                                     __   '