blame: make diff algorithm configurable

The diff algorithm used in 'git-blame(1)' is set to 'myers',
without the possibility to change it aside from the `--minimal` option.

There has been long-standing interest in changing the default diff
algorithm to "histogram", and Git 3.0 was floated as a possible occasion
for taking some steps towards that:

https://lore.kernel.org/git/xmqqed873vgn.fsf@gitster.g/

As a preparation for this move, it is worth making sure that the diff
algorithm is configurable where useful.

Make it configurable in the `git-blame(1)` command by introducing the
`--diff-algorithm` option and make honor the `diff.algorithm` config
variable. Keep Myers diff as the default.

Signed-off-by: Antonin Delpeuch <antonin@delpeuch.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/diff-options.adoc

@@ -197,26 +197,7 @@ and starts with _<text>_, this algorithm attempts to prevent it from
 appearing as a deletion or addition in the output. It uses the "patience
 diff" algorithm internally.
 
-`--diff-algorithm=(patience|minimal|histogram|myers)`::
-	Choose a diff algorithm. The variants are as follows:
-+
---
-   `default`;;
-   `myers`;;
-	The basic greedy diff algorithm. Currently, this is the default.
-   `minimal`;;
-	Spend extra time to make sure the smallest possible diff is
-	produced.
-   `patience`;;
-	Use "patience diff" algorithm when generating patches.
-   `histogram`;;
-	This algorithm extends the patience algorithm to "support
-	low-occurrence common elements".
---
-+
-For instance, if you configured the `diff.algorithm` variable to a
-non-default value and want to use the default one, then you
-have to use `--diff-algorithm=default` option.
+include::diff-algorithm-option.adoc[]
 
 `--stat[=<width>[,<name-width>[,<count>]]]`::
 	Generate a diffstat. By default, as much space as necessary