Use system-installed git binary for push/pull/fetch (#25900)

### Problem

When using HTTPS remotes, users are getting errors when trying to push
or pull via the git panel.

On macOS, Zed bundles a `git` binary that's part of
[`dugite-native`](https://github.com/desktop/dugite-native). But we
don't include the entire package. Additional binaries from
`dugite-native` are needed for pulling and pushing over HTTPS.

### Solution

Rather than bundling those additional binaries, I've changed the `push`,
`pull`, and `fetch` actions to rely on the *system-installed* `git`
binary. The downside of this is that, if the user does not have Git
installed, they wont' be able to push, pull, or fetch from within Zed.
But we believe that the vast majority of users will have Git installed.
Also, unlike `diff` and `status`, which Zed needs to call in the
background without any user interaction, `push`/`pull` and `fetch` are
explicit actions that the user takes in Zed, so there is an opportunity
to prompt them to install Git if they haven't.

### Background

There are three ways (that I know of) that users might authenticate when
pushing, pulling, or fetching over HTTPS.

1. Via a built-in [Git
`credential.helper`](https://git-scm.com/docs/gitcredentials). On macOS,
Git ships with a helper called `credential-osxkeychain` that stores
internet passwords in the OS Keychain. You can opt into this globally
with the command `git config --global credential.helper osxkeychain`,
which writes to your `~/.gitconfig`.
2. Via [`Git Credential Manager`
(GCM)](https://github.com/git-ecosystem/git-credential-manager), which
is a different `credential.helper`, [built by
GitHub](https://github.blog/security/application-security/git-credential-manager-authentication-for-everyone/),
which must be installed manually, and integrates with specific Git
hosting providers like GitHub and Azure.
3. By typing their Username and Password/Access-token interactively when
pushing/pulling/fetching.

### Testing Status

* [ ] 🚫 Interactive password auth - not yet supported, requires
https://github.com/zed-industries/zed/pull/25848
* [x] **credential-osxkeychain** - when using the built-in credential
helper, and the credentials are already stored in the keychain,
push/pull/fetch now work fine .
* [ ] **GCM**  - still testing.
* Right now, I'm seeing `git-credential-manager` just hang indefinitely
when pushing from Zed, even though it works when pushing from a
terminal.



Release Notes:

- N/A

crates/git/src/repository.rs

@@ -614,7 +614,7 @@ impl GitRepository for RealGitRepository {
     ) -> Result<()> {
         let working_directory = self.working_directory()?;
 
-        let output = new_std_command(&self.git_binary_path)
+        let output = new_std_command("git")
             .current_dir(&working_directory)
             .args(["push", "--quiet"])
             .args(options.map(|option| match option {
@@ -638,7 +638,7 @@ impl GitRepository for RealGitRepository {
     fn pull(&self, branch_name: &str, remote_name: &str) -> Result<()> {
         let working_directory = self.working_directory()?;
 
-        let output = new_std_command(&self.git_binary_path)
+        let output = new_std_command("git")
             .current_dir(&working_directory)
             .args(["pull", "--quiet"])
             .arg(remote_name)
@@ -658,7 +658,7 @@ impl GitRepository for RealGitRepository {
     fn fetch(&self) -> Result<()> {
         let working_directory = self.working_directory()?;
 
-        let output = new_std_command(&self.git_binary_path)
+        let output = new_std_command("git")
             .current_dir(&working_directory)
             .args(["fetch", "--quiet", "--all"])
             .output()?;