worktree: factor out repeated string literal

worktree: factor out repeated string literal

For each worktree removed by "git worktree prune", it reports the reason
for the removal. All reasons share the common prefix "Removing
worktrees/%s:". As new removal reasons are added, this prefix needs to
be duplicated, which is error-prone and potentially cumbersome.
Therefore, factor out the common prefix.

Although this change seems to increase the "sentence lego quotient", it
should be reasonably safe, as the reason for removal is a distinct
clause, not strictly related to the prefix. Moreover, the "worktrees" in
"Removing worktrees/%s:" is a path literal which ought not be localized,
so by factoring it out, we can more easily avoid exposing that path
fragment to translators.

Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>