utils/git: use exact format for last_revision_*

Format for oneline can be overridden in user's gitconfig. If that's the
case, last_revision_commit_of_file won't work properly, and because of
that last_revision_of_file won't work at all.

Here's what I was getting when running brew tests:

       expected: "6bec2de"
            got: "\e[33m6bec2dee633f\e[m"

I have abbrev length set to 12, and oneline formatting is more colorful.

So, instead of relying on overrideable format, it should rather specify
"--format=%h" to get only hash, and speciy --abbrev=7 to force abbrev
length to be 7

After this commit tests are passing for me.

Library/Homebrew/utils/git.rb

@@ -8,9 +8,10 @@ module Git
 
     out, = Open3.capture3(
       HOMEBREW_SHIMS_PATH/"scm/git", "-C", repo,
-      "log", "--oneline", "--max-count=1", *args, "--", file
+      "log", "--format=%h", "--abbrev=7", "--max-count=1",
+      *args, "--", file
     )
-    out.split(" ").first
+    out.chomp
   end
 
   def last_revision_of_file(repo, file, before_commit: nil)