setup_git_env(): introduce git_path_from_env() helper

"Check the value of an environment and fall back to a known path
inside $GIT_DIR" is repeated a few times to determine the location
of the data store, the index and the graft file, but the return
value of getenv is not guaranteed to survive across further
invocations of setenv or even getenv.

Make sure to xstrdup() the value we receive from getenv(3), and
encapsulate the pattern into a helper function.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
Merge branch 'je/pager-do-not-recurse' into maint

We used to unconditionally disable the pager in the pager process
we spawn to feed out output, but that prevented people who want to
run "less" within "less" from doing so.

* je/pager-do-not-recurse:
  pager: do allow spawning pager recursively
Merge branch 'mn/sideband-no-ansi' into maint

Tools that read diagnostic output in our standard error stream do
not want to see terminal control sequence (e.g. erase-to-eol).
Detect them by checking if the standard error stream is connected
to a tty.

* mn/sideband-no-ansi:
  sideband.c: do not use ANSI control sequence on non-terminal
Merge branch 'jk/report-fail-to-read-objects-better' into maint

Reworded the error message given upon a failure to open an existing
loose object file due to e.g. permission issues; it was reported as
the object being corrupt, but that is not quite true.

* jk/report-fail-to-read-objects-better:
  open_sha1_file: report "most interesting" errno
Merge branch 'jc/revision-dash-count-parsing' into maint

"git log -2master" is a common typo that shows two commits starting
from whichever random branch that is not 'master' that happens to
be checked out currently.

* jc/revision-dash-count-parsing:
  revision: parse "git log -<count>" more carefully
Merge branch 'as/pretty-truncate' into maint

The "%<(10,trunc)%s" pretty format specifier in the log family of
commands is used to truncate the string to a given length (e.g. 10
in the example) with padding to column-align the output, but did
not take into account that number of bytes and number of display
columns are different.

* as/pretty-truncate:
  pretty.c: format string with truncate respects logOutputEncoding
  t4205, t6006: add tests that fail with i18n.logOutputEncoding set
  t4205 (log-pretty-format): use `tformat` rather than `format`
  t4041, t4205, t6006, t7102: don't hardcode tested encoding value
  t4205 (log-pretty-formats): don't hardcode SHA-1 in expected outputs
Merge branch 'ow/config-mailmap-pathname' into maint

The "mailmap.file" configuration option did not support the tilde
expansion (i.e. ~user/path and ~/path).

* ow/config-mailmap-pathname:
  config: respect '~' and '~user' in mailmap.file
Merge branch 'jk/complete-merge-pull' into maint

The completion scripts (in contrib/) did not know about quite a few
options that are common between "git merge" and "git pull", and a
couple of options unique to "git merge".

* jk/complete-merge-pull:
  completion: add missing options for git-merge
  completion: add a note that merge options are shared
Merge branch 'jc/apply-ignore-whitespace' into maint

"--ignore-space-change" option of "git apply" ignored the spaces
at the beginning of line too aggressively, which is inconsistent
with the option of the same name "diff" and "git diff" have.

* jc/apply-ignore-whitespace:
  apply --ignore-space-change: lines with and without leading whitespaces do not match
Merge branch 'jx/blame-align-relative-time' into maint

"git blame" miscounted number of columns needed to show localized
timestamps, resulting in jaggy left-side-edge of the source code
lines in its output.

* jx/blame-align-relative-time:
  blame: dynamic blame_date_width for different locales
  blame: fix broken time_buf paddings in relative timestamp
Merge branch 'bc/blame-crlf-test' into maint

"git blame" assigned the blame to the copy in the working-tree if
the repository is set to core.autocrlf=input and the file used CRLF
line endings.

* bc/blame-crlf-test:
  blame: correctly handle files regardless of autocrlf
Merge branch 'jk/commit-C-pick-empty' into maint

"git commit --allow-empty-message -C $commit" did not work when the
commit did not have any log message.

* jk/commit-C-pick-empty:
  commit: do not complain of empty messages from -C
Merge branch 'jk/diff-files-assume-unchanged' into maint

"git diff --find-copies-harder" sometimes pretended as if the mode
bits have changed for paths that are marked with assume-unchanged
bit.

* jk/diff-files-assume-unchanged:
  run_diff_files: do not look at uninitialized stat data
Merge branch 'jk/diff-follow-must-take-one-pathspec' into maint

"git format-patch" did not enforce the rule that the "--follow"
option from the log/diff family of commands must be used with
exactly one pathspec.

* jk/diff-follow-must-take-one-pathspec:
  move "--follow needs one pathspec" rule to diff_setup_done
Merge branch 'nd/daemonize-gc' into maint

"git gc --auto" was recently changed to run in the background to
give control back early to the end-user sitting in front of the
terminal, but it forgot that housekeeping involving reflogs should
be done without other processes competing for accesses to the refs.

* nd/daemonize-gc:
  gc --auto: do not lock refs in the background
Merge branch 'sk/spawn-less-case-insensitively-from-grep-O-i' into maint

"git grep -O" to show the lines that hit in the pager did not work
well with case insensitive search.  We now spawn "less" with its
"-I" option when it is used as the pager (which is the default).

* sk/spawn-less-case-insensitively-from-grep-O-i:
  git grep -O -i: if the pager is 'less', pass the '-I' option
Merge branch 'nd/index-pack-one-fd-per-thread' into maint

We used to disable threaded "git index-pack" on platforms without
thread-safe pread(); use a different workaround for such
platforms to allow threaded "git index-pack".

* nd/index-pack-one-fd-per-thread:
  index-pack: work around thread-unsafe pread()
Merge branch 'jk/index-pack-report-missing' into maint

The error reporting from "git index-pack" has been improved to
distinguish missing objects from type errors.

* jk/index-pack-report-missing:
  index-pack: distinguish missing objects from type errors
Merge branch 'rs/mailinfo-header-cmp' into maint

"git mailinfo" used to read beyond the end of header string while
parsing an incoming e-mail message to extract the patch.

* rs/mailinfo-header-cmp:
  mailinfo: use strcmp() for string comparison
Merge branch 'dt/merge-recursive-case-insensitive' into maint

On a case insensitive filesystem, merge-recursive incorrectly
deleted the file that is to be renamed to a name that is the same
except for case differences.

* dt/merge-recursive-case-insensitive:
  mv: allow renaming to fix case on case insensitive filesystems
  merge-recursive.c: fix case-changing merge bug