Isolating test from accessing global git credential helper config (#16895)

## Summary
Resolves: https://github.com/astral-sh/uv/issues/1980

Added a utility function to TestContext called
**with_git_credential_helper_blocked** that isolates the test from
accessing the credential helper value defined in global/system git
config. It does so, by writing to a file .gitconfig in the temporary
home_dir that is created as part of the TestContext.

## Test Plan
Tested it by running the test
pip_install::install_git_private_https_pat_and_username, and making sure
it doesn't affect the keyring.

## Note:
The commit hash for the uv-private-package seems to have changed.
Kindly, ensure that the modification related to that is correct.

crates/uv/tests/it/common/mod.rs

@@ -12,10 +12,13 @@ use uv_python::downloads::ManagedPythonDownloadList;
 
 use assert_cmd::assert::{Assert, OutputAssertExt};
 use assert_fs::assert::PathAssert;
-use assert_fs::fixture::{ChildPath, PathChild, PathCopy, PathCreateDir, SymlinkToFile};
+use assert_fs::fixture::{
+    ChildPath, FileWriteStr, PathChild, PathCopy, PathCreateDir, SymlinkToFile,
+};
 use base64::{Engine, prelude::BASE64_STANDARD as base64};
 use futures::StreamExt;
 use indoc::formatdoc;
+use indoc::indoc;
 use itertools::Itertools;
 use predicates::prelude::predicate;
 use regex::Regex;
@@ -563,6 +566,19 @@ impl TestContext {
         self
     }
 
+    // Unsets the git credential helper using temp home gitconfig
+    pub fn with_unset_git_credential_helper(self) -> Self {
+        let git_config = self.home_dir.child(".gitconfig");
+        git_config
+            .write_str(indoc! {r"
+                [credential]
+                    helper =
+            "})
+            .expect("Failed to unset git credential helper");
+
+        self
+    }
+
     /// Clear filters on `TestContext`.
     pub fn clear_filters(mut self) -> Self {
         self.filters.clear();

crates/uv/tests/it/pip_install.rs

@@ -2501,14 +2501,10 @@ fn install_git_private_https_pat_at_ref() {
 
 /// Install a package from a private GitHub repository using a PAT and username
 /// An arbitrary username is supported when using a PAT.
-///
-/// TODO(charlie): This test modifies the user's keyring.
-/// See: <https://github.com/astral-sh/uv/issues/1980>.
 #[test]
 #[cfg(feature = "git")]
-#[ignore = "Modifies the user's keyring"]
 fn install_git_private_https_pat_and_username() {
-    let context = TestContext::new(DEFAULT_PYTHON_VERSION);
+    let context = TestContext::new(DEFAULT_PYTHON_VERSION).with_unset_git_credential_helper();
     let token = decode_token(common::READ_ONLY_GITHUB_TOKEN);
     let user = "astral-test-bot";
 
@@ -2522,7 +2518,7 @@ fn install_git_private_https_pat_and_username() {
     Resolved 1 package in [TIME]
     Prepared 1 package in [TIME]
     Installed 1 package in [TIME]
-     + uv-private-pypackage==0.1.0 (from git+https://astral-test-bot:****@github.com/astral-test/uv-private-pypackage@6c09ce9ae81f50670a60abd7d95f30dd416d00ac)
+     + uv-private-pypackage==0.1.0 (from git+https://astral-test-bot:****@github.com/astral-test/uv-private-pypackage@d780faf0ac91257d4d5a4f0c5a0e4509608c0071)
     "###);
 
     context.assert_installed("uv_private_pypackage", "0.1.0");