completion: let 'for-each-ref' filter remote branches for 'checkout' …

…DWIMery

The code listing unique remote branches for 'git checkout's tracking
DWIMery outputs only remote branches that match the current word to be
completed, but the filtering is done in a shell loop iterating over
all remote refs.

Let 'git for-each-ref' do the filtering, as it can do so much more
efficiently and we can remove that shell loop entirely.

This speeds up refs completion for 'git checkout' considerably when
there are a lot of non-matching remote refs to be filtered out.
Uniquely completing a branch in a repository with 100k remote
branches, all packed, best of five:

  On Linux, before:

    $ time __git_complete_refs --cur=maste --track

    real    0m1.993s
    user    0m1.740s
    sys     0m0.304s

  After:

    real    0m0.266s
    user    0m0.248s
    sys     0m0.012s

  On Windows, before:

    real    0m6.187s
    user    0m3.358s
    sys     0m2.121s

  After:

    real    0m0.750s
    user    0m0.015s
    sys     0m0.090s

Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -422,15 +422,9 @@ __git_refs ()
 			# employ the heuristic used by git checkout
 			# Try to find a remote branch that matches the completion word
 			# but only output if the branch name is unique
-			local ref entry
-			__git for-each-ref --shell --format="ref=%(refname:strip=3)" \
-				"refs/remotes/" | \
-			while read -r entry; do
-				eval "$entry"
-				if [[ "$ref" == "$match"* ]]; then
-					echo "$ref"
-				fi
-			done | sort | uniq -u
+			__git for-each-ref --format="%(refname:strip=3)" \
+				"refs/remotes/*/$match*" "refs/remotes/*/$match*/**" | \
+			sort | uniq -u
 		fi
 		return
 	fi