completion: extract function __git_dwim_remote_heads

__git_refs() has the ability to report unique remote names for
supporting completion of remote branch names for the DWIMery of git
checkout and git switch.

For git checkout, this is fine, because it always supports completing
all local references.

However, git switch by default only supports either switching branches
or using this DWIMery to create a local branch tracking the remote
branch.

Future work to cleanup and improve completion support for git switch
will be aided if the remote branch names can be completed separately
from __git_refs.

Extract this logic to a function __git_dwim_remote_heads(), and use it
in __git_refs.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -621,6 +621,26 @@ __git_tags ()
 			"refs/tags/$cur_*" "refs/tags/$cur_*/**"
 }
 
+# List unique branches from refs/remotes used for 'git checkout' and 'git
+# switch' tracking DWIMery.
+# 1: A prefix to be added to each listed branch (optional)
+# 2: List only branches matching this word (optional; list all branches if
+#    unset or empty).
+# 3: A suffix to be appended to each listed branch (optional).
+__git_dwim_remote_heads ()
+{
+	local pfx="${1-}" cur_="${2-}" sfx="${3-}"
+	local fer_pfx="${pfx//\%/%%}" # "escape" for-each-ref format specifiers
+
+	# employ the heuristic used by git checkout and git switch
+	# Try to find a remote branch that cur_es the completion word
+	# but only output if the branch name is unique
+	__git for-each-ref --format="$fer_pfx%(refname:strip=3)$sfx" \
+		--sort="refname:strip=3" \
+		"refs/remotes/*/$cur_*" "refs/remotes/*/$cur_*/**" | \
+	uniq -u
+}
+
 # Lists refs from the local (by default) or from a remote repository.
 # It accepts 0, 1 or 2 arguments:
 # 1: The remote to list refs from (optional; ignored, if set but empty).
@@ -696,13 +716,7 @@ __git_refs ()
 		__git_dir="$dir" __git for-each-ref --format="$fer_pfx%($format)$sfx" \
 			"${refs[@]}"
 		if [ -n "$track" ]; then
-			# 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
-			__git for-each-ref --format="$fer_pfx%(refname:strip=3)$sfx" \
-				--sort="refname:strip=3" \
-				"refs/remotes/*/$match*" "refs/remotes/*/$match*/**" | \
-			uniq -u
+			__git_dwim_remote_heads "$pfx" "$match" "$sfx"
 		fi
 		return
 	fi