show-branch: convert per-branch flags to commit-slab

show-branch uses commit->object.flags to store per-branch
reachability bits, one bit per branch starting at REV_SHIFT.
The flags word has only a fixed number of available bits, limiting
the number of branches that can be shown simultaneously.

Convert the per-branch bits to a dedicated commit-slab using uint64_t
as the element type, initialized with a stride via
init_commit_rev_flags_with_stride(). Keep the UNINTERESTING bit in
object.flags where it belongs, as it is used for revision walking and
does not need to be in the per-branch slab. With UNINTERESTING removed
from the slab, REV_SHIFT becomes 0 and all 64 bits of uint64_t are
available for branch tracking.

Add helper functions get_rev_flags_ptr(), peek_rev_flags_ptr(),
has_any_rev_flags(), or_rev_flag_bit(), test_rev_flag_bit(),
has_all_rev_flags(), has_only_rev_flag_bit(), and
has_subset_rev_flags() to encapsulate per-bit slab access cleanly.
Use has_only_rev_flag_bit() in show_independent() to preserve the
original semantics: a commit is independent only if reachable from
exactly one tip. Use has_subset_rev_flags() in join_revs() to skip
parents whose flags already include all of the current commit's flags,
avoiding redundant propagation and queue thrashing in dense histories.

Fix join_revs() to correctly propagate UNINTERESTING to parents using
a local commit_is_merge_base variable, without smudging the commit
itself, matching the original behavior.

Remove the hardcoded MAX_REVS limit entirely by dynamically growing
the ref_name[] array with ALLOC_GROW, and allocating rev[] and
reflog_msg[] dynamically after all refs are collected. Compute
flags_stride at runtime as (ref_name_cnt + 63) / 64 so the slab
supports an arbitrary number of branches.

Remove the hardcoded branch limit from documentation since there is
no longer a fixed maximum. Add tests to verify show-branch works
correctly with more than 27 branches. Include revision.h for the
shared UNINTERESTING definition.

Signed-off-by: Gatla Vishweshwar Reddy <gatlavishweshwarreddy26@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: Gatla Vishweshwar Reddy

Documentation/git-show-branch.adoc

@@ -22,8 +22,6 @@ Shows the commit ancestry graph starting from the commits named
 with <rev>s or <glob>s (or all refs under refs/heads
 and/or refs/tags) semi-visually.
 
-It cannot show more than 26 branches and commits at a time.
-
 It uses `showbranch.default` multi-valued configuration items if
 no <rev> or <glob> is given on the command line.