Use anyhow more idiomatically (#31052)

https://github.com/zed-industries/zed/issues/30972 brought up another
case where our context is not enough to track the actual source of the
issue: we get a general top-level error without inner error.

The reason for this was `.ok_or_else(|| anyhow!("failed to read HEAD
SHA"))?; ` on the top level.

The PR finally reworks the way we use anyhow to reduce such issues (or
at least make it simpler to bubble them up later in a fix).
On top of that, uses a few more anyhow methods for better readability.

* `.ok_or_else(|| anyhow!("..."))`, `map_err` and other similar error
conversion/option reporting cases are replaced with `context` and
`with_context` calls
* in addition to that, various `anyhow!("failed to do ...")` are
stripped with `.context("Doing ...")` messages instead to remove the
parasitic `failed to` text
* `anyhow::ensure!` is used instead of `if ... { return Err(...); }`
calls
* `anyhow::bail!` is used instead of `return Err(anyhow!(...));`

Release Notes:

- N/A

crates/fs/src/fake_git_repo.rs

@@ -1,5 +1,5 @@
 use crate::FakeFs;
-use anyhow::{Context as _, Result, anyhow};
+use anyhow::{Context as _, Result};
 use collections::{HashMap, HashSet};
 use futures::future::{self, BoxFuture};
 use git::{
@@ -80,7 +80,7 @@ impl GitRepository for FakeGitRepository {
                 state
                     .index_contents
                     .get(path.as_ref())
-                    .ok_or_else(|| anyhow!("not present in index"))
+                    .context("not present in index")
                     .cloned()
             })
             .await
@@ -95,7 +95,7 @@ impl GitRepository for FakeGitRepository {
                 state
                     .head_contents
                     .get(path.as_ref())
-                    .ok_or_else(|| anyhow!("not present in HEAD"))
+                    .context("not present in HEAD")
                     .cloned()
             })
             .await
@@ -119,8 +119,8 @@ impl GitRepository for FakeGitRepository {
         _env: Arc<HashMap<String, String>>,
     ) -> BoxFuture<anyhow::Result<()>> {
         self.with_state_async(true, move |state| {
-            if let Some(message) = state.simulated_index_write_error_message.clone() {
-                return Err(anyhow!("{}", message));
+            if let Some(message) = &state.simulated_index_write_error_message {
+                anyhow::bail!("{message}");
             } else if let Some(content) = content {
                 state.index_contents.insert(path, content);
             } else {