* jk/git-prompt:
contrib: add credential helper for OS X Keychain
Makefile: OS X has /dev/tty
Makefile: linux has /dev/tty
credential: use git_prompt instead of git_getpass
prompt: use git_terminal_prompt
add generic terminal prompt function
refactor git_getpass into generic prompt function
move git_getpass to its own source file
imap-send: don't check return value of git_getpass
imap-send: avoid buffer overflow