Get only patch version updates to imara-diff

This configures Dependabot version updates to keep `imara-diff` at
0.1.*. It does not affect Dependabot security updates, nor other
ways of updating besides Dependabot, nor other crate dependencies
if they are unrelated to `imara-diff`. See:

- https://github.blog/changelog/2021-05-21-dependabot-version-updates-can-now-ignore-major-minor-patch-releases/
- https://docs.github.com/en/code-security/dependabot/working-with-dependabot/dependabot-options-reference#ignore--

This is by the same technique as used in:
GitoxideLabs/cargo-smart-release@aeb91ee

See discussion in #2068 for details for why we are not upgrading
`imara-diff` to 0.2 at this time.

Author: EliahKagan

.github/dependabot.yml

@@ -9,6 +9,12 @@ updates:
       prefix: ''
     allow:
       - dependency-type: all
+    ignore:
+      # Keep imara-diff at 0.1.* for now (see comments in #2068).
+      - dependency-name: imara-diff
+        update-types:
+          - 'version-update:semver-major'
+          - 'version-update:semver-minor'
     groups:
       cargo:
         patterns: ['*']