Complete bug IDs where appropriate

This completes bug IDs in bash and fish.  I haven't gotten this to work in zsh.
In fish, the bug titles are shown as completion label, and can be searched for.
I don't know if/how other shells can show the completion label.
Maybe they don't use "\t" as separator but it seems to do no harm.

The interface does not yet seem ideal because sometimes subcommands and
bug IDs are both valid. Since there can be lots of bug IDs, they overshadow
the subcommands.
I wonder if instead of

	git bug status open 0123acd

we should use

	git bug status --open 0123acd

or if there's some other way to avoid ambiguity.

If that doesn't work out, fish has a -k/--keep-order flag, so subcommand
completions can be listed before bug IDs, but that's neither portable nor
a pretty solution.

Part of #493

commands/comment.go

@@ -19,6 +19,7 @@ func newCommentCommand() *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runComment(env, args)
 		},
+		ValidArgsFunction: completeBugs(env),
 	}
 
 	cmd.AddCommand(newCommentAddCommand())

commands/helper_completion.go

@@ -0,0 +1,54 @@
+package commands
+
+import (
+	"log"
+
+	"github.com/spf13/cobra"
+
+	"github.com/MichaelMure/git-bug/cache"
+	"github.com/MichaelMure/git-bug/query"
+)
+
+type validArgsFunction func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective)
+
+func completeBugs(env *Env) validArgsFunction {
+	return func(cmd *cobra.Command, args []string, toComplete string) ([]string, cobra.ShellCompDirective) {
+		// Currently, the commands accept at most one bug ID.
+		if len(args) != 0 {
+			return nil, cobra.ShellCompDirectiveNoFileComp
+		}
+
+		if err := loadBackend(env)(cmd, args); err != nil {
+			log.Println(err)
+			return nil, cobra.ShellCompDirectiveError
+		}
+		defer func() {
+			err := closeBackend(env)(cmd, args)
+			if err != nil {
+				log.Println(err)
+			}
+		}()
+
+		q := query.NewQuery()
+		allIds, err := env.backend.QueryBugs(q)
+		if err != nil {
+			log.Println(err)
+			return nil, cobra.ShellCompDirectiveError
+		}
+
+		bugExcerpt := make([]*cache.BugExcerpt, len(allIds))
+		for i, id := range allIds {
+			bugExcerpt[i], err = env.backend.ResolveBugExcerpt(id)
+			if err != nil {
+				log.Println(err)
+				return nil, cobra.ShellCompDirectiveError
+			}
+		}
+
+		completions := make([]string, len(allIds))
+		for i, id := range allIds {
+			completions[i] = id.Human() + "\t" + bugExcerpt[i].Title
+		}
+		return completions, cobra.ShellCompDirectiveDefault
+	}
+}

commands/label.go

@@ -17,6 +17,7 @@ func newLabelCommand() *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runLabel(env, args)
 		},
+		ValidArgsFunction: completeBugs(env),
 	}
 
 	cmd.AddCommand(newLabelAddCommand())

commands/rm.go

@@ -18,6 +18,7 @@ func newRmCommand() *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runRm(env, args)
 		},
+		ValidArgsFunction: completeBugs(env),
 	}
 
 	flags := cmd.Flags()

commands/select.go

@@ -32,6 +32,7 @@ The complementary command is "git bug deselect" performing the opposite operatio
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runSelect(env, args)
 		},
+		ValidArgsFunction: completeBugs(env),
 	}
 
 	return cmd

commands/show.go

@@ -30,6 +30,7 @@ func newShowCommand() *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runShow(env, options, args)
 		},
+		ValidArgsFunction: completeBugs(env),
 	}
 
 	flags := cmd.Flags()

commands/status.go

@@ -16,6 +16,7 @@ func newStatusCommand() *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runStatus(env, args)
 		},
+		ValidArgsFunction: completeBugs(env),
 	}
 
 	cmd.AddCommand(newStatusCloseCommand())

commands/title.go

@@ -16,6 +16,7 @@ func newTitleCommand() *cobra.Command {
 		RunE: func(cmd *cobra.Command, args []string) error {
 			return runTitle(env, args)
 		},
+		ValidArgsFunction: completeBugs(env),
 	}
 
 	cmd.AddCommand(newTitleEditCommand())

misc/bash_completion/git-bug

@@ -740,6 +740,7 @@ _git-bug_comment()
 
     must_have_one_flag=()
     must_have_one_noun=()
+    has_completion_function=1
     noun_aliases=()
 }
 
@@ -822,6 +823,7 @@ _git-bug_label()
 
     must_have_one_flag=()
     must_have_one_noun=()
+    has_completion_function=1
     noun_aliases=()
 }
 
@@ -1002,6 +1004,7 @@ _git-bug_rm()
 
     must_have_one_flag=()
     must_have_one_noun=()
+    has_completion_function=1
     noun_aliases=()
 }
 
@@ -1022,6 +1025,7 @@ _git-bug_select()
 
     must_have_one_flag=()
     must_have_one_noun=()
+    has_completion_function=1
     noun_aliases=()
 }
 
@@ -1052,6 +1056,7 @@ _git-bug_show()
 
     must_have_one_flag=()
     must_have_one_noun=()
+    has_completion_function=1
     noun_aliases=()
 }
 
@@ -1114,6 +1119,7 @@ _git-bug_status()
 
     must_have_one_flag=()
     must_have_one_noun=()
+    has_completion_function=1
     noun_aliases=()
 }
 
@@ -1181,6 +1187,7 @@ _git-bug_title()
 
     must_have_one_flag=()
     must_have_one_noun=()
+    has_completion_function=1
     noun_aliases=()
 }