patchImprovement to only call Git Credential Helper once

>
> When calling the Git Credential Helper that is set in the git config,
> the get command can return a credential. Git immediately turns around
> and calls the store command, even though that credential was just
> retrieved by the Helper. This creates two side effects. First of all,
> if the Helper requires a passphrase, the user has to type it in
> twice. Secondly, if the user has a number of helpers, this retrieves
> the credential from one service and writes it to all services.
>
> This commit introduces a new field in the credential struct that
> detects when the credential was retrieved using the Helper, and early
> exits when called to store the credential.
> ---
>  credential.c | 8 +++++++-
>  credential.h | 3 ++-
>  2 files changed, 9 insertions(+), 2 deletions(-)
>
> diff --git a/credential.c b/credential.c
> index 62be651b0..79bf62d49 100644
> --- a/credential.c
> +++ b/credential.c
> @@ -280,8 +280,10 @@ void credential_fill(struct credential *c)
>
>   for (i = 0; i < c->helpers.nr; i++) {
>   credential_do(c, c->helpers.items[i].string, "get");
> - if (c->username && c->password)
> + if (c->username && c->password) {
> + c->retrieved = 1;
>   return;
> + }
>   if (c->quit)
>   die("credential helper '%s' told us to quit",
>       c->helpers.items[i].string);
> @@ -300,6 +302,10 @@ void credential_approve(struct credential *c)
>   return;
>   if (!c->username || !c->password)
>   return;
> + if (c->retrieved) {
> + c->approved = 1;
> + return;
> + }
>
>   credential_apply_config(c);
>
> diff --git a/credential.h b/credential.h
> index 6b0cd16be..d99df2f52 100644
> --- a/credential.h
> +++ b/credential.h
> @@ -8,7 +8,8 @@ struct credential {
>   unsigned approved:1,
>   configured:1,
>   quit:1,
> - use_http_path:1;
> + use_http_path:1,
> + retrieved:1;
>
>   char *username;
>   char *password;