Fix updating pins on crates which track Alire-generated files

src/alire/alire-user_pins.adb

@@ -199,7 +199,7 @@ package body Alire.User_Pins is
          Trace.Detail ("Checking out pin " & Utils.TTY.Name (Crate) & " at "
                        & TTY.URL (Destination));
 
-         --  If the fetch URL has been changed, fall back to checkout
+         --  If the fetch URL has been changed, do a fresh 'git clone'.
          --
          --  Note that VCSs.Git.Clone converts the URL to a git-friendly form
          --  with VCSs.Repo, so this is what the output of 'git config' should
@@ -216,6 +216,12 @@ package body Alire.User_Pins is
             return;
          end if;
 
+         --  Discard any uncommitted changes (e.g. files in 'config/' which
+         --  Alire has automatically generated).
+         --  These can cause Update's 'git pull' command to fail.
+
+         VCSs.Git.Discard_Uncommitted (Repo => Destination).Assert;
+
          --  Finally update. In case the branch has just been changed by the
          --  user in the manifest, the following call will also take care of
          --  it.

src/alire/alire-vcss-git.adb

@@ -292,6 +292,21 @@ package body Alire.VCSs.Git is
          return Alire.Errors.Get (E);
    end Commit_All;
 
+   -------------------------
+   -- Discard_Uncommitted --
+   -------------------------
+
+   function Discard_Uncommitted (Repo : Directory_Path) return Outcome
+   is
+      Guard : Directories.Guard (Directories.Enter (Repo)) with Unreferenced;
+   begin
+      Run_Git (Empty_Vector & "reset" & "--hard" & "HEAD");
+      return Outcome_Success;
+   exception
+      when E : others =>
+         return Alire.Errors.Get (E);
+   end Discard_Uncommitted;
+
    ----------
    -- Push --
    ----------

src/alire/alire-vcss-git.ads

@@ -104,6 +104,9 @@ package Alire.VCSs.Git is
    --  Add and commit all changes in a given repo; commiter will be set to the
    --  user email stored in our config.
 
+   function Discard_Uncommitted (Repo : Directory_Path) return Outcome;
+   --  Reset all uncommitted changes to tracked files
+
    function Push (Repo   : Directory_Path;
                   Remote : String;
                   Force  : Boolean := False;

testsuite/tests/pin/branch-update-dirty/test.py

@@ -0,0 +1,54 @@
+"""
+Check updating a branch pin when 'config/*' files are tracked.
+"""
+
+
+import os
+import subprocess
+
+from drivers.alr import run_alr, alr_pin, init_local_crate
+from drivers.asserts import assert_match, assert_in_file
+from drivers.helpers import git_branch, git_commit_file, init_git_repo
+
+
+# Create crate yyy, with git tracking the 'config/*' files
+init_local_crate("yyy")
+yyy_path = os.getcwd()
+with open(".gitignore") as f:
+    gitignore_content = f.read()
+gitignore_content = gitignore_content.replace("/config/\n", "")
+with open(".gitignore", "w") as f:
+    f.write(gitignore_content)
+init_git_repo(".")
+default_branch = git_branch()
+os.chdir("..")
+
+# Create and build another crate, with yyy's default branch added as a pin
+init_local_crate()
+xxx_path = os.getcwd()
+alr_pin("yyy", url=f"git+file:{yyy_path}", branch=default_branch)
+run_alr("build")
+
+# Verify that the cached copy of yyy has a dirty repo (due to changes to the
+# 'config/*' files)
+cached_yyy_path = os.path.join(xxx_path, "alire", "cache", "pins", "yyy")
+os.chdir(cached_yyy_path)
+p = subprocess.run(["git", "status"], capture_output=True)
+assert_match(r".*modified:\s*config/yyy_config\.gpr", p.stdout.decode())
+
+# Add commits to yyy's default branch, including a change to a file in 'config/'
+os.chdir(yyy_path)
+git_commit_file(
+    "Change_config", "config/yyy_config.gpr", "This is a new addition\n", "a"
+)
+git_commit_file("Add_test_file", "test_file", "This is a new file\n")
+
+# Check that the dirty repo doesn't prevent updating the pin
+os.chdir(xxx_path)
+run_alr("update")
+
+# Check that the update was successful
+assert_in_file(os.path.join(cached_yyy_path, "test_file"), "This is a new file")
+
+
+print('SUCCESS')

testsuite/tests/pin/branch-update-dirty/test.yaml

@@ -0,0 +1,3 @@
+driver: python-script
+indexes:
+    compiler_only_index: {}