forgit: Add interactive git discard selector

Currently, discarding uncommitted changes requires two separate git
commands: git reset HEAD to unstage files and git restore to revert
working tree changes. This two-step process is error-prone and
cumbersome when dealing with multiple files, especially since there is
no built-in interactive selector for this combined operation.

Add forgit_discard (gdc) command that combines unstage and restore into
a single atomic operation with interactive file selection. This
streamlines the workflow for discarding changes while maintaining the
safety and visibility benefits of interactive selection through fzf.
The command includes a warning in documentation about its destructive
nature to prevent accidental data loss.

Signed-off-by: Javier Tia <floss@jetm.me>

Author: jetm

README.md

@@ -146,6 +146,13 @@ If you're having issues after updating, and commands such as `forgit::add` or al
 
 - **Interactive `git restore <file>` selector** (`grs`)
 
+- **Interactive `git discard <file>` selector** (`gdc`)
+
+  Combines `git reset HEAD` (unstage) and `git restore` (revert) into a single operation,
+  fully discarding all changes for selected files back to their committed state.
+
+  > **Warning:** This permanently discards uncommitted changes with no way to recover them.
+
 - **Interactive `git checkout <file>` selector** (`gcf`)
 
 - **Interactive `git checkout <branch>` selector** (`gcb`)
@@ -225,6 +232,7 @@ forgit_show=gso
 forgit_add=ga
 forgit_reset_head=grh
 forgit_restore=grs
+forgit_discard=gdc
 forgit_ignore=gi
 forgit_attributes=gat
 forgit_checkout_file=gcf
@@ -290,6 +298,7 @@ These are passed to the according `git` calls.
 | `gso`     | `FORGIT_SHOW_GIT_OPTS`                                                      |
 | `grh`    | `FORGIT_RESET_HEAD_GIT_OPTS`                                                |
 | `grs`    | `FORGIT_RESTORE_GIT_OPTS`                                                   |
+| `gdc`    | `FORGIT_DISCARD_GIT_OPTS`                                                   |
 | `gcf`    | `FORGIT_CHECKOUT_FILE_GIT_OPTS`                                             |
 | `gcb`    | `FORGIT_CHECKOUT_BRANCH_GIT_OPTS`, `FORGIT_CHECKOUT_BRANCH_BRANCH_GIT_OPTS` |
 | `gsw`    | `FORGIT_SWITCH_BRANCH_GIT_OPTS`                                             |
@@ -351,6 +360,7 @@ Customizing fzf options for each command individually is also supported:
 | `gso`    | `FORGIT_SHOW_FZF_OPTS`            |
 | `grh`    | `FORGIT_RESET_HEAD_FZF_OPTS`      |
 | `grs`    | `FORGIT_RESTORE_FZF_OPTS`         |
+| `gdc`    | `FORGIT_DISCARD_FZF_OPTS`         |
 | `gcf`    | `FORGIT_CHECKOUT_FILE_FZF_OPTS`   |
 | `gcb`    | `FORGIT_CHECKOUT_BRANCH_FZF_OPTS` |
 | `gsw`    | `FORGIT_SWITCH_BRANCH_FZF_OPTS`   |