This reverts commit 31b49d9b.

That commit taught do_askpass to hand ownership of our
buffer back to the caller rather than simply return a
pointer into our internal strbuf.  What it failed to notice,
though, was that our internal strbuf is static, because we
are trying to emulate the getpass() interface.

By handing off ownership, we created a memory leak that
cannot be solved. Sometimes git_prompt returns a static
buffer from getpass() (or our smarter git_terminal_prompt
wrapper), and sometimes it returns an allocated string from
do_askpass.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

The current askpass code simply dies if calling an askpass
helper fails. Worse, in some failure modes it doesn't even
print an error (if start_command fails, then it prints its
own error; if reading fails, we print an error; but if the
command exits non-zero, finish_command fails and we print
nothing!).

Let's be more kind to the user by printing an error message
when askpass doesn't work out, and then falling back to the
terminal (which also may fail, of course, but we die already
there with a nice message).

While we're at it, let's clean up the existing error
messages a bit.  Now that our prompts are very long and
contain quotes and colons themselves, our error messages are
hard to read.

So the new failure modes look like:

  [before, with a terminal]
  $ GIT_ASKPASS=false git push
  $ echo $?
  128

  [before, with no terminal, and we must give up]
  $ setsid git push
  fatal: could not read 'Password for 'https://peff@github.com': ': No such device or address

  [after, with a terminal]
  $ GIT_ASKPASS=false git push
  error: unable to read askpass response from 'false'
  Password for 'https://peff@github.com':

  [after, with no terminal, and we must give up]
  $ GIT_ASKPASS=false setsid git push
  error: unable to read askpass response from 'false'
  fatal: could not read Password for 'https://peff@github.com': No such device or address
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

The do_askpass function inherited a few bad habits from the
original git_getpass. One, there's no need to strbuf_reset a
buffer which was just initialized. And two, it's a good
habit to use strbuf_detach to claim ownership of a buffer's
string (even though in this case the owning buffer goes out
of scope, so it's effectively the same thing).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Our custom implementation of git_terminal_prompt has many
advantages over regular getpass(), as described in the prior
commit.

This also lets us implement a PROMPT_ECHO flag for callers
who want it.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

This will allow callers to specify more options (e.g.,
leaving echo on). The original git_getpass becomes a slim
wrapper around the new function.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

This is currently in connect.c, but really has nothing to
do with the git protocol itself. Let's make a new source
file all about prompting the user, which will make it
cleaner to refactor.
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>