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/git/src/status.rs

@@ -1,5 +1,5 @@
 use crate::repository::RepoPath;
-use anyhow::{Result, anyhow};
+use anyhow::Result;
 use serde::{Deserialize, Serialize};
 use std::{path::Path, str::FromStr, sync::Arc};
 use util::ResultExt;
@@ -241,7 +241,7 @@ impl StatusCode {
             b'R' => Ok(StatusCode::Renamed),
             b'C' => Ok(StatusCode::Copied),
             b' ' => Ok(StatusCode::Unmodified),
-            _ => Err(anyhow!("Invalid status code: {byte}")),
+            _ => anyhow::bail!("Invalid status code: {byte}"),
         }
     }
 
@@ -286,7 +286,7 @@ impl UnmergedStatusCode {
             b'A' => Ok(UnmergedStatusCode::Added),
             b'D' => Ok(UnmergedStatusCode::Deleted),
             b'U' => Ok(UnmergedStatusCode::Updated),
-            _ => Err(anyhow!("Invalid unmerged status code: {byte}")),
+            _ => anyhow::bail!("Invalid unmerged status code: {byte}"),
         }
     }
 }