credential: add new interactive config option

credential: add new interactive config option

When scripts or background maintenance wish to perform HTTP(S) requests,
there is a risk that our stored credentials might be invalid. At the
moment, this causes the credential helper to ping the user and block the
process. Even if the credential helper does not ping the user, Git falls
back to the 'askpass' method, which includes a direct ping to the user
via the terminal.

Even setting the 'core.askPass' config as something like 'echo' will
causes Git to fallback to a terminal prompt. It uses
git_terminal_prompt(), which finds the terminal from the environment and
ignores whether stdin has been redirected. This can also block the
process awaiting input.

Create a new config option to prevent user interaction, favoring a
failure to a blocked process.

The chosen name, 'credential.interactive', is taken from the config
option used by Git Credential Manager to already avoid user
interactivity, so there is already one credential helper that integrates
with this option. However, older versions of Git Credential Manager also
accepted other string values, including 'auto', 'never', and 'always'.
The modern use is to use a boolean value, but we should still be
careful that some users could have these non-booleans. Further, we
should respect 'never' the same as 'false'. This is respected by the
implementation and test, but not mentioned in the documentation.

The implementation for the Git interactions takes place within
credential_getpass(). The method prototype is modified to return an
'int' instead of 'void'. This allows us to detect that no attempt was
made to fill the given credential, changing the single caller slightly.

Also, a new trace2 region is added around the interactive portion of the
credential request. This provides a way to measure the amount of time
spent in that region for commands that _are_ interactive. It also makes
a conventient way to test that the config option works with
'test_region'.

Signed-off-by: Derrick Stolee <stolee@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>