Clean up Git info a bit

Remove an extraneous newline, and pass `--git-dir` to `git` to ensure that
out-of-repo builds don't accidentally pick up a parent repo's info.

preproc/src/git.rs

@@ -20,7 +20,13 @@ pub struct Commit {
 impl Commit {
     pub fn rev_parse(what: &str) -> Result<Self, Error> {
         let output = Command::new("git")
-            .args(["show", "-s", "--format=%H%x00%h%x00%ci", what])
+            .args([
+                "--git-dir=.git",
+                "show",
+                "-s",
+                "--format=%H%x00%h%x00%ci",
+                what,
+            ])
             .stderr(Stdio::inherit())
             .stdin(Stdio::null())
             .output()
@@ -31,15 +37,18 @@ impl Commit {
                 output.status
             )));
         }
-        let info = String::from_utf8(output.stdout).expect("Commit info is not valid UTF-8??");
+        let mut info = String::from_utf8(output.stdout).expect("Commit info is not valid UTF-8??");
+        let trimmed_len = info.trim_end().len();
+        info.truncate(trimmed_len);
 
         let first_split = info
             .find('\0')
             .expect("Failed to split hash and short hash");
         let second_split = info[first_split + 1..]
             .find('\0')
             .expect("Failed to split short hash and timestamp")
-            + first_split;
+            + first_split
+            + 1;
 
         Ok(Self {
             info,