Add pacman if-branch for Git Bash

[yanorei32]

This patch fixes pacman: command not found issue in Git for Windows.

This PR is separated. ( #758 and #759 )

Please refer Packages section of #606 (comment)

Expected patched (#758 + this) behaviour in Git Bash

curl -sL https://github.com/yanorei32/screenFetch/raw/msys-cygwin-patch-combined/screenfetch-dev | bash

Expected patched (this PR only) behaviour in Git Bash

curl -sL https://github.com/yanorei32/screenFetch/raw/fix-git-for-windows-pacman-issue/screenfetch-dev | bash

Actual behaviour in Git Bash