git-core: Support retrieving passwords with GIT_ASKPASS

git-core: Support retrieving passwords with GIT_ASKPASS

git tries to read a password from the terminal in imap-send and
when talking to a http server that requires authentication.

When a GUI is driving git, however, the end user is not paying
attention to the terminal (there may not even be a terminal).
GUI would appear to hang forever.

Fix this problem by allowing a password-retrieving command
to be specified in GIT_ASKPASS

Signed-off-by: Frank Li <lznuaa@gmail.com>
Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>