fix: Skip EXEPATH optimization if it finds no best path

The `EXEPATH` optimization for `gix_path::env::system_prefix()` on
Windows previously tried https://www.msys2.org/docs/environments
prefixes used by Git for Windows, returning a successful result on
the first subdirectory found. However, if `EXEPATH` points to a
directory that contains more than one such subdirectory, then the
subdirectory returned would not necessarily be the correct one.

Getting more than one subdirectory is unlikely. It is expected that
at most one of `clangarm64`, `mingw64`, and `mingw32` will be
present. However, there are at least three ways it could happen:

- `EXEPATH` has the intended meaning as the root of a Git for
  Windows installation, but the directory it refers to contains
  multiple directories left over from a previous installation. A
  corresponding scenario applies to `ALTERNATIVE_LOCATIONS`, but
  it is resolved by checking all the directories in a reasonable
  order. Here, we are not checking the contents of the directories,
  so no matter what order we look for them in, picking one when
  there are others risks picking the wrong one.

- `EXEPATH` has the intended meaning as the root of a Git for
  Windows installation, but it is a custom installation produced
  from local build or custom distribution rather than an official
  build, and it contains multiple such directories because it
  carries binaries built against more than one of the targets (even
  if only one of them has `git` itself). A corresponding scenario
  is fairly unlikely for `ALTERNATIVE_LOCATIONS`, because a custom
  MSYS2 tree, even if created using the Git for Windows SDK, would
  probably not be installed in one of the common Git for Windows
  installation locations, without considering the effects of doing
  so. In contrast, `EXEPATH` will often be set in a Git Bash
  environment even in a highly customized Git for Windows tree.

- `EXEPATH` has a different meaning from what is intended. For
  example, the user might set it to the root of an ordinary MSYS2
  installation. (Note that it is also likely to have various other
  meanings even more different from these, but those won't likely
  cause the `EXEPATH` optimization to be used when it shouldn't,
  because most possible meanings of `EXEPATH` won't involve a
  subdirectory of any of the names we look for.

Instead of using the first existing subdirectory we fine, this
checks for all of them and requires that exactly one exist. If more
than one exist, then that is now treated the same as if none exist,
falling back to the `git --exec-path`-based strategy, which is
sometimes slower but more robust.

Author: EliahKagan

gix-path/src/env/mod.rs

@@ -155,13 +155,17 @@ where
     F: Fn(&str) -> Option<OsString>,
 {
     let root = PathBuf::from(var_os_func("EXEPATH")?);
-    for candidate in ["clangarm64", "mingw64", "mingw32"] {
-        let candidate = root.join(candidate);
-        if candidate.is_dir() {
-            return Some(candidate);
-        }
+
+    let mut candidates = ["clangarm64", "mingw64", "mingw32"]
+        .iter()
+        .map(|component| root.join(component))
+        .filter(|candidate| candidate.is_dir());
+
+    let path = candidates.next()?;
+    match candidates.next() {
+        Some(_) => None, // Multiple plausible candidates, so don't use the `EXEPATH` optimization.
+        None => Some(path),
     }
-    None
 }
 
 /// Returns the platform dependent system prefix or `None` if it cannot be found (right now only on Windows).