RFR: 8365048: idea.sh script does not correctly detect/handle git worktrees

[David Beaumont]

Allows files or directories called .git to indicate likely use of Git for VCS in IntelliJ.
Tested locally in a worktree and normal repository. It produces:

 <component name="VcsDirectoryMappings">
    <mapping directory="$PROJECT_DIR$" vcs="Git" />
    <mapping directory="$PROJECT_DIR$/open" vcs="Git" />
  </component>

in both cases, as expected.

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Commit messages:
 - Simple fix (just checking file)

Changes: https://git.openjdk.org/jdk/pull/26777/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26777&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8365048
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/26777.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26777/head:pull/26777

PR: https://git.openjdk.org/jdk/pull/26777