Re: [PATCH 19/20] abbrev: support relative abbrev values

> diff --git a/Documentation/config.txt b/Documentation/config.txt
> index ab641bf5a9..abf07be7b6 100644
> --- a/Documentation/config.txt
> +++ b/Documentation/config.txt
> @@ -919,6 +919,12 @@ core.abbrev::
>         in your repository, which hopefully is enough for
>         abbreviated object names to stay unique for some time.
>         The minimum length is 4.
> ++
> +This can also be set to relative values such as `+2` or `-2`, which
> +means to add or subtract N characters from the SHA-1 that Git would
> +otherwise print, this allows for producing more future-proof SHA-1s
> +for use within a given project, while adjusting the value for the
> +current approximate number of objects.

> diff --git a/Documentation/diff-options.txt b/Documentation/diff-options.txt
> index f466600972..f1114a7b8d 100644
> --- a/Documentation/diff-options.txt
> +++ b/Documentation/diff-options.txt
> @@ -384,6 +384,9 @@ endif::git-format-patch[]
>         independent of the `--full-index` option above, which controls
>         the diff-patch output format.  Non default number of
>         digits can be specified with `--abbrev=<n>`.
> ++
> +Can also be set to a relative value, see `core.abbrev` in
> +linkgit:git-diff[1].

> diff --git a/config.c b/config.c
> index 12f762ad92..cd95c6bdfb 100644
> --- a/config.c
> +++ b/config.c
> @@ -1151,6 +1151,17 @@ static int git_default_core_config(const char *var, const char *value)
>                         return config_error_nonbool(var);
>                 if (!strcasecmp(value, "auto")) {
>                         default_abbrev = -1;
> +               } else if (*value == '+' || *value == '-') {
> +                       int relative = git_config_int(var, value);
> +                       if (relative == 0)
> +                               die(_("bad core.abbrev value %s. "

> +                                     "relative values must be non-zero"),
> +                                   value);
> +                       if (abs(relative) > GIT_SHA1_HEXSZ)
> +                               die(_("bad core.abbrev value %s. "
> +                                     "impossibly out of range"),
> +                                   value);
> +                       default_abbrev_relative = relative;
>                 } else {
>                         int abbrev = git_config_int(var, value);
>                         if (abbrev < minimum_abbrev || abbrev > 40)
> diff --git a/diff.c b/diff.c
> index e0141cfbc0..f7861b8472 100644
> --- a/diff.c
> +++ b/diff.c
> @@ -4801,16 +4801,28 @@ int diff_opt_parse(struct diff_options *options,
>         else if (!strcmp(arg, "--abbrev"))
>                 options->abbrev = DEFAULT_ABBREV;
>         else if (skip_prefix(arg, "--abbrev=", &arg)) {
> +               int v;
>                 char *end;
>                 if (!strcmp(arg, ""))
>                         die("--abbrev expects a value, got '%s'", arg);
> -               options->abbrev = strtoul(arg, &end, 10);
> +               v = strtoul(arg, &end, 10);
>                 if (*end)
>                         die("--abbrev expects a numerical value, got '%s'", arg);
> -               if (options->abbrev < MINIMUM_ABBREV) {
> +               if (*arg == '+' || *arg == '-') {
> +                       if (v == 0) {
> +                               die("relative abbrev must be non-zero");
> +                       } else if (abs(v) > the_hash_algo->hexsz) {
> +                               die("relative abbrev impossibly out of range");
> +                       } else {
> +                               default_abbrev_relative = v;
> +                               options->abbrev = DEFAULT_ABBREV;
> +                       }
> +               } else if (v < MINIMUM_ABBREV) {
>                         options->abbrev = MINIMUM_ABBREV;
> -               } else if (the_hash_algo->hexsz < options->abbrev) {
> +               } else if (the_hash_algo->hexsz < v) {
>                         options->abbrev = the_hash_algo->hexsz;
> +               } else {
> +                       options->abbrev = v;