Fix Git commit ID detection in worktrees

When using a Git worktree, ".git" is a file, not a directory, hence "configure" doesn't detect it correctly.

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Build System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture

changed milestone to %8.6.1

changed weight to 5

added Tbug Trac import labels

added 1 deleted label

closed

Merged with 2a162eba.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedFixed

removed 1 deleted label

The new Hadrian build system has been merged. Relabeling the tickets concerning the legacy make build system to prevent confusion.

Trac metadata

Trac field	Value
Component	Build System → Build System (make)

added Pnormal label