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Check if Zsh is installed before installing anything else.
New-commers, or people who don't read the disclaimers(ex. me) will go
straight to stack overflow if chsh -s returns an error.

fix test for chsh in install
in case $SHELL is zsh but not the last one in /etc/shells
fixes #3026 fixes #3779 fixes #3780

install.sh: Replace nonstandard "sed -i" with sed + mv commands. Makes portable to systems whose sed lacks "-i", like OpenBSD.

Use `--depth=1` to clone just the lastest version rather than every version in git.

Some environments (such as Android) does not have /usr/bin.

The intention of the redirection to /dev/null is to hide the output
'hash: no such command: git' since we rely on the exit status.

However, the output goes to stderr, so it's stderr that needs to be
redirected. For completeness, we redirect both stderr and stdout using
'2>&1'.

Example:

  [~]$ hash git > /dev/null
  [~]$ PATH=''
  [~]$ hash git > /dev/null
  hash: no such command: git
  [~]$ hash git > /dev/null 2>&1
  [~]$

Fixing a few quirks in the latest installer updates (quoting /Users/robbyrussell/.oh-my-zsh and such). Also mentioining our twitter account after install

Although the zshrc template adds a PATH= string, this is overwritten by the installer script.
This allows it to be placed anywhere in the file instead of having to append at the end.

Fix finding git issue. http://stackoverflow.com/questions/592620/check-if-a-program-exists-from-a-bash-script

Make tools/install.sh and tools/upgrade.sh pretty.

Updating install process to copy your current environments PATH and adding it to the bottom of ~/.zshrc.