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/git.rs

@@ -7,7 +7,7 @@ pub mod status;
 
 pub use crate::hosting_provider::*;
 pub use crate::remote::*;
-use anyhow::{Context as _, Result, anyhow};
+use anyhow::{Context as _, Result};
 pub use git2 as libgit;
 use gpui::action_with_deprecated_aliases;
 use gpui::actions;
@@ -99,7 +99,7 @@ impl FromStr for Oid {
 
     fn from_str(s: &str) -> std::prelude::v1::Result<Self, Self::Err> {
         libgit::Oid::from_str(s)
-            .map_err(|error| anyhow!("failed to parse git oid: {}", error))
+            .context("parsing git oid")
             .map(Self)
     }
 }